Biodegradable gelatin-aminodextran particle coatings of and processes for making same

ABSTRACT

The invention relates generally to colloidal particle having a core material and a gelatin/aminodextran coating with pendent functional groups attached thereto. Biological substances or molecules, especially monoclonal antibodies, may be attached to said particles. The monoclonal antibody containing particles are useful in a variety of positive and negative biological assays.

RELATED INVENTION

This application is a division of application Ser. No. 07/968, 158,filed Oct. 29, 1992, now U.S. Pat. No. 5,466,609, issued Nov. 14, 1995,which is a continuation-in-part of application Ser. No. 07/607,253,filed Oct. 31, 1990, now U.S. Pat. No. 5,169,754, issued Dec. 8, 1992.This application also is related to application Ser. No. 07/827,347,filed Jan. 29, 1992, entitled FORMATION OF COLLOIDAL METAL DISPERSIONSUSING AMINODEXTRANS AS REDUCTANTS AND PROTECTIVE AGENTS, now U.S. Pat.No. 5,248,772, issued Sep. 28, 1993; and to application Ser. No.07/532,434, filed Jun. 4, 1990, and entitled IN SITU USE OF GELATIN INTHE PREPARATION OF UNIFORM FERRITE PARTICLES, now U.S. Pat. No.5,062,991, issued Nov. 5, 1991. The specifications of these copendingapplications and patent are incorporated herein by reference. Thisapplication and these copending applications and patent are owned by acommon assignee.

FIELD OF THE INVENTION

This invention relates generally to colloidal sized particles having acrosslinked gelatin or aminodextran coating with pendent functionalgroups attached thereto. Specifically, this invention relates tocolloidal particles having a crosslinked gelatin or aminodextran coatingthat is functionalized to bind a pendant protein such as an antibody, tothe method of making such particles and to the use of such particles inbiological assays.

BACKGROUND OF THE INVENTION

The use of polymeric particles and magnetic particles to bind a compoundhas long been known and used in industrial and laboratory procedures.For example, the Merrifield resins, crosslinked styrene-divinylbenzenespheroidal beads, were among the earliest and most widely used modernsubstrate particles. They were used in organic synthesis, forheterogenizing homogeneous catalysts and in biochemical reactions. Sincethe Merrifield resins were fairly large, they could easily be separatedby filtration. In some fields, however, it is desirable to use colloidalsized particles because the material to be bound is scarce, expensive oris to be used in a procedure where larger particles are not desirable.This is particularly true in the biochemical field. When particles areof colloidal size, however, their separation from liquid medium byfiltration can become lengthy and difficult. In particular, colloidalparticles tend to coat the surface of the filter and slow the process.The use of magnetic particles, specifically magnetic particles having apolymeric coating, has found great utility because such particles can bemagnetically gathered to one side of a reaction vessel and the bulk ofthe reaction medium simply decanted. (The word "particles" as usedherein encompasses spheres, spheroids, beads and other shapes as well.These words are used interchangeably unless otherwise specified.) Theuse of coated magnetic particles has found a particular utility inbiological applications, especially where antibodies are bound to thesurface coating of the particles. The bound antibodies may then be usedto capture a specific biological substance from a test sample containingnumerous biological samples or to capture undesired species from thetest sample, leaving the desired species in the sample.

The categories of coated magnetic particles, also known as magneticspheres or beads, can be divided into four general classes.

1. Core-and-shell beads with a magnetic core and a hard shell coating ofpolymerized monomer or a silanizing agent. See U.S. Pat. No. 4,267,234to Rembaum (polyglutaraldehyde shell around ferrofluid core particles);U.S. Pat. No. 4,454,234 to Czerlinski (suspension or emulsionpolymerized coating around submicron magnetic particles); U.S. Pat. Nos.4,554,088, 4,695,392 and 4,695,393 to Whitehead et al. (silanizedmagnetic oxide particles of polydisperse size and shape); U.S. Pat. No.4,672,040 to Josephson (polysilane coated magnetic particles); U.S. Pat.No. 4,783,336 to Margel et al. (suspension polymerized polyacroleinaround ferrofluid particles); U.S. Pat. No. 4,795,698 to Owen et al.(bovine serum albumin coating); and U.S. Pat. No. 4,964,007 to yudelson(gelatin-gum arabic-surfactant coating);

2. Core-and-shell beads with a magnetic core and a loose shell of randomcoil or globular polymer which may or may not be crosslinked. See U.S.Pat. No. 4,452,773 to Molday (dextran coating around ferrofluidparticles) and U.S. Pat. No. 4,795,698 to Owen et al. (protein such asbovine serum albumin around ferrofluid particles.

3. Magnetic latex materials formed by uniformly embedding ferrofluidparticles in polystyrene latex particles. See U.S. Pat. No. 4,358,388 toDaniel et al.

4. Porous polymer particles filled with magnetic materials such aspolymer-ferrite or polymer maghemite composite systems. See K. Nustad etal. "Monodisperse Polymer Particles In Immunoassays And CellSeparation", Microspheres: Medical and Biological Applications, A.Rembaum and Z. Tokes, eds. (Boca Raton, Fla.: CRC Press, 1988) pages53-75; C.D. Platsoucas et al., "The Use of Magnetic Monosized PolymerParticles For The Removal Of T Cells From Human Bone Marrow CellSuspensions", ibid. at pages 89-99; and U.S. Pat. Nos. 4,563,510,4,530,956 and 4,654,267 International Patent Publication No. WO83/03920! to Ughelstad et al. (polymer coated magnetic particlesprepared by treating compact or porous particles with a solution of ironsalts and the use of such particles for medical, diagnostic or otherpurposes).

The usefulness of most polymer coated magnetic beads in medical andbiological applications has been limited by practical considerationssuch as the uniformity of particle size and shape, the need for thebiological reagent to be strongly bound to the particle, a preferencefor hydrophilic polymer coatings as opposed to hydrophobic coatings, andwhether or not the coating is biodegradable. While biodegradability isof particular importance where a biological reagent is to administeredin vivo, it is also important in various cell sorting, separation andassay procedures. The most desirable coated magnetic particles wouldhave the following features.

1. The particles should be as small as possible in order to maximize thesurface area on which the biological reagent is coated, but theparticles should still be easily separable with a small magnet. Smallsize and large surface area are desirable in order to use the leastpossible quantity of particles to remove the targeted substance; e.g.,to interact with on the order of 10⁶ cells per sample in one step,thereby avoiding sequential additions and work-ups.

2. There should be a low non-specific binding of the antibody-coatedparticles to cell surfaces. The particle surface should be hydrophilicor covered with a coating of a hydrophilic substance to which theantibody is attached.

3. The polymer and antibody layers on the particles should be covalentlybound to each other in order to reduce dissociation and conformationalchanges.

4. The coating on the magnetic particles and any molecular chains whichlink an antibody to the polymer surface should be metabolizable.

5. In positive selection of cells, a mechanism for quickly and easilyrecovering viable cells from the magnetic particles should be availablein order that recovered cells can be cultured.

6. In the negative selection of cells, the antibody-coated particlesshould be sterile so that the remaining cells can be cultured.

7. For magnetic separation and sorting of cells and other biologicalsubstances, the preferred magnetic particles are "soft" magneticparticles. That is, particles which can be easily magnetized enddemagnetized as opposed to hard or permanent magnetic. The particles canbe ferromagnetic, ferrimagnetic or superparamagnetic. Ferromagnetic andferrimagnetic particles are not limited in size, whereassuperparamagnetic particles are limited to single domain structures ofdimensions usually less than about 40 nanometers. (C. Kittel et al.,Solid State Physics 3:437-464 (1956).

Problems exist with using each of the magnetic-composite particles fromeach of the above class in cell separation procedures. Some examples ofthe problems encountered are:

1. Ferrofluid core and the usual polymer outer shell particles have toosmall a magnetic moment to make them practical for use in cellseparations where hand-held permanent magnets are used to collect andseparate the magnetic particles. Such particles require the usehigh-field separation techniques which severely limits the volume ofmaterial which can be processed, thus limiting scale-up.

2. Ferrofluid-polystyrene particles prepared by emulsion polymerizationcannot be tightly controlled in size and range from about 0.1 to 4 μm indiameter. Consequently, in cell separations using antibodies conjugatedto such beads, the very small, kinetically-mobile magnetic particleswhich inherently possess the least magnetic moment tend topreferentially occupy the antigenic sates on a cell surface. As aresult, the resulting cell-bead conjugates do not have a sufficient notmagnetic moment to permit easy separation.

The use of magnetic particles having first and second layers of types Band A gelatin, respectively, and prepared as taught herein and in thecopending priority application Ser. No. 07/607,253, overcomes thesedifficulties. However, gelatin coated particles have been found to havesome problems regarding non-specific interactions with certain cells,notably platelets and phagocyte cells such as monocytes. The problemarises because the amino acid sequence of gelatin (as exemplified by theα-1 chain of rat and calf skin collagen) includes three regions with thetripeptide sequence Arg-Gly-Asp (RGD) which duplicates the RGD bindingsequence of fibronectin, a component of the extracellular matrix thatspecifically promotes cellular adhesion. Those biological cells withfibronectin expressed on their surface have a specific affinity forcollagen, which is equivalent to crosslinked gelatin. For example,antibody containing gelatin coated magnetic ferrite particles used inthe separation of subsets of white blood cells will also bind tofibronectin on the surface of platelets and monocytes. The result isnon-specific depletion of cells because monocytes and platelets arebound to the particles as well as those cells which bearantibody-specific antigens. The non-specific depletion of cells can beavoided through the use of an aminodextran as the outermost coatinglayer on coated particles. The use of dextran derivatives as carriershas been discussed by U. Manabe et el., J. Lab. Clin. Med. 104:445-454(1984) (antibody-polyaldehyde dextran-methotrexate); L. B. Shin et al.,Intl. J. Cancer 41:832-839 (1988) (antibody-aminodextran-methotrexate);A. R. Oseroff et al., Proc. Natl. Acad. Sci. USA 83:8744-8748 (1986)(antibody-aminodextran-chlorine 6); S. Rakestraw et al., Proc, Natl.,Acad. Sci. USA 87:4217-4221 (1990) (antibody-dextran hydrazide-Sn(IV)chlorine 6); R. J. Mrsnay et al., Eur. J. Cell. Biol. 45: 200-208 (1987)(ouabain-aminodextran-gold particle); J. W. M. Bulte et al., MagneticRes. 25:148-157 (1992) (anti particle); and other as described in S.S.Wong, "Chemistry of Protein Conjugation and Cross-Linking" (CRC Press,Boca Raton, Fla. 1991).

The various particles described above have been used in the biologicalarts to immobilize a variety of biological substances, particularlyantibodies. In using such particles, immobilization of antibodies bycovalent coupling is preferred to immobilization by antibody adsorptionwhich requires careful and separate adjustment of pH and antibodyconcentration for each monoclonal antibody used. P. Bagchi et al., J.Colloid Interface Sci., 83:460-478 (1981); J. Lyklema, Colloids andSurfaces, 10:33-42 (1984); M. D. Bale et al., J. Colloid Interface Sci.,125:516-525 (1988); C. C. Ho et al., ibid., 121:564-570 (1988);"Proteins at Interfaces: Physicochemical and Biochemical Studies", ACSSymposium Series, No. 343, J. L. Brash and T. A. Horbett, Eds.(Washington: Amer. Chem. Soc., 1987); W. Norde, Adv. Coll. InterfaceSci., 25:267-340 (1986); A. V. Elgersma et al., Abstracts of the 198thAmer. Chem. Soc. Meeting, Miami Beach, Fla., Sep. 10-15, 1989, COLL0131; and D. E. Brooks, Annenberg Center for Health Sciences and H. B.Wallis Research Facility at Eisenhower Latex Conference, Orlando, Fla.,Dec. 4-5, 1989. However, even when the pH and antibody are carefullycontrolled, there is little assurance that the orientation of adsorbedantibody will be such that an active adsorbed antibody will result.Adsorbed antibodies also have long term storage problems arising fromantibody desorption from the particles' surfaces. Furthermore, proteins,such as antibodies, tend to achieve maximum adsorption on hydrophobicsurfaces at or near the pI of the protein. However, if electrostaticinteractions between charge groups are important, then the adsorbingsurface and the adsorbate should have net opposite charges. Covalentcoupling methods, on the other hand, are not as sensitive to theseconditions.

Covalent coupling methods have been used with particles of magnetiteembedded in carboxy-modified latex subsequently coated with aminodextranR. S. Molday et al. FEBS. Lett., 170:232-238 (1984)! and derivatizedwith a number of antibodies as described in copending application Ser.No. 07/255,743 which is incorporated herein by reference. If theantibody is of IgG isotype, the covalent coupling method assures thatthe linkage between the antibody and the particles occurs at theantibody Fc or hinge region, and not at the antibody's Fab region. Ifthe antibody is of pentameric IgM isotype which has only Fab regionsexposed, the coupling of one Fab region to the particle will still leavefour Fab regions exposed and available for reaction.

This invention provides for the preparation of magnetic particles havinga biodegradable coating to which can be attached pendent biologicalsubstances, such as monoclonal antibodies. The particles of theinvention can be used in various cell separation and assaymethodologies. Biodegradability in the coating used on the magnetic corematerial is important in cell separation technology. For example,antibodies may be conjugated to gelatin/aminodextran coated magneticparticles such as manganese ferrite particles. These particles wouldcontain a coating and a manganese-iron oxide core, all of which arebiodegradable. In a positive cell selection procedure using suchparticles, once the desired cell has been isolated from other cells, theparticles and coating can be allowed to degrade in a manner such thatthe cells are kept viable and can be cultured for further use.Alternatively, the enzyme collagenase can be used first to release thecore material (magnetic or latex) by digestion of thegelatin/aminodextran coating. The core material can then be removed fromthe cell suspension before culturing the cells. In the negativeselection of cells with such biodegradable beads, the beads can be leftin the cell suspension from which targeted cells were removed withoutcompromising the viability of the remaining cells. For example, in bonemarrow purging operations using biodegradable magnetic beads, there isless concern about leaving behind some beads in the purged marrow thatis to be transplanted in a patient. Currently, syntheticpolymer-magnetite particles prepared by Ughelstad et al, U.S. Pat. No.4,654,267 (WO 83/03920), and conjugated with antibody are being used inbone marrow purging. The polymer is not biodegradable and imparts ahydrophobic surface to these beads. This hydrophobicity, which is notpresent in the gelatin/aminodextran coated particles of the claimedinvention, is responsible for non-specific interactions between thebeads and cells. As a result of this non-specific interaction, theselectivity is poor and more beads must be used to attain the desiredlevel of treatment. The claimed invention avoids these problems.

SUMMARY OF THE INVENTION

The invention provides discrete colloidal particles having a solid coreand coated with a first layer of a water soluble gelatin and a secondlayer of an aminodextran, said coating being crosslinked or fixed by theaction of a chemical crosslinking agent and having a plurality ofpendent functional groups. The pendent functional groups may be or haveterminal aldehyde or carboxylate groups, amine groups, sulfhydryl groupsor maleimidyl groups, and polyclonal or monoclonal antibodies. The coremay be metallic particles formed in gelatin solution or preformedparticles which are then coated with the gelatin.

The invention provides discrete colloidal particles having pendentbiological functional groups such as polyclonel and monoclonalantibodies covalently attached to the crosslinked second aminodextranlayer by means of a heterobifunctional crosslinking agent so as toenable advantageous use of said antibody functionalized particles inbiological separations and assays. The heterobifunctional crosslinkingagent acts as a bridging group between the biological substance orfunctional group and the crosslinked gelatin or aminodextran.

The invention provides a process for the preparation of discretecolloidal particles having a solid core coated with a biodegradablefirst layer of crosslinked gelatin and biodegradable second layer of anaminodextran having pendent functional groups. The process comprisescoating a solid core material which has a hydrophobic surface with firstgelatin layer and a second aminodextran layer, crosslinking the adsorbedouter coating and derivatizing the crosslinked coating to obtain aproduct having a desired reactive species covalently bound to saidcrosslinked coating surface. The invention further provides a method forthe preparation of particle bound polyclonal and monoclonal antibodies.

The invention provides a process for the separation, either positive ornegative, and analysis of biological substances comprising contacting asolution containing a biological substance with an antibody covalentlybound to the surface of a crosslinked gelatin/aminodextran coated solidcore particle, incubating the resultant mixture at a temperature and fora time sufficient to form a complex between said antibody and saidsubstance, separating the particles from the solution and analyzing theparticles or the solution for the presence and/or absence of the desiredsubstance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the number of neutrophils depleted versus the magneticbead-to-cell ratio.

FIGS. 2A-2L show a distinct and progressive shift toward less forwardscatter (FS) or smaller size and more side scatter (LSS) or greatergranularity in the normal granulocyte region as higher titers ofmagnetic beads were added to cell samples.

FIG. 3 is a plot of the number of red blood cells depleted versus themagnetic bead-to-cell ratio.

FIG. 4 is a plot of the number of depleted cells versus the magenticbead-to-cell ratio.

FIG. 5 is a plot of the number of white blood cells depleted versus themagnetic bead-to-cell ratio.

DETAILED DESCRIPTION OF THE INVENTION

In the Detailed Description Of The Invention and Preferred Embodimentswhich follow, applicants place reactive maleimidyl groups on thecrosslinked gelatin and/or aminodextran coated particles and reactivesulfhydryl groups on the antibodies. These may be reversed such that themaleimidyl groups are attached to the antibodies and the sulfhydrylgroups are attached to the crosslinked gelatin and/or aminodextran.Applicants have also elected to use 2-iminothiolane hydrochloride as themodel for the sulfhydryl reagent and sulfo-SMCC (described below) as themodel for the maleimidyl reagent. Other reagents enumerated or of likenature and result may also be used.

Glossary of Biological Reagents

All of the monoclonal antibodies (Ab) referred to herein are identifyingdesignations used by Coulter Corporation, Hialeah, Fla. for monoclonalantibodies made by Coulter Corporation. The following informationfurther identifies the antibodies used herein. The use of thesemonoclonal antibodies is by way of example only and is not to beunderstood as limiting the invention. The term "CD" refers to "ClusterDesignation" adopted by the International Workshops on Buman LeukocyteDifferentiation Antigens. A.T.C.C. is the American Type CultureCollection, Rockville, Md.

    ______________________________________                                        Antibody CD       Description or Reference                                    ______________________________________                                        T11      CD2      Derived from hybridization of mouse                                           NS/1-AG4 cells with spleen cells of                                           BALB/cJ mice immunized with T cell                                            chronic lymphocytic leukemia cells.                         T4       CD4      As T11, but immunized with peripheral                                         human T lymphocytes.                                        T8       CD8      As T11, but immunized with human                                              thymocytes.                                                 KC16     --       U.S. Pat. No. 4,752,563;                                                      A.T.C.C. Deposit No. CRL 8994.                              1D3      --       U.S. Pat. No. 4,931,395;                                                      A.T.C.C. Deposit No. HB 9445                                KC48     --       U.S. Pat. No. 4,865,971;                                                      A.T.C.C. Deposit No. HB 9584                                MO2      CD14     R. F. Todd et al.                                                             J. Immunol., 126:1435 (1981).                               PLT-1    CD41     R. F. Todd et al., Blood, 59:775 (1982);                                      Griffith et al., Blood, 61:85 (1983).                       KC56     CD45     Derived from hybridization of mouse                                           Sp2/O-AG14 myeloma cells with spleen                                          from BALB/c mice immunized with a                                             derivative of the CEM cell line.                            ______________________________________                                    

Other reagents used herein and commercially obtainable from CoulterCorporation are:

MsIgG1-RD1/MsIgG1-FITC: Mouse IgGl-phycoerythrin RD1!/ MouseIgG1-Fluorescein Iso-thiocyanate FITC!.

T11-RD1/B4-FITC: Ab T11-phycoerythrin/Ab B4-FITC.

T4-RD1/T8-FITC: Ab T4-phycoerythrin/Ab T8-FITC.

1X PBS Dissolve 53.8 g K₂ HPO₄ in 1.6 L distilled water. Add 12.8 g KH₂PO₄ and stir until dissolved. Then dissolve 340 g NaCl in the solution.After all salts dissolved, add distilled water to make up to 2 L volumeand filter through a 0.2 μm filter. The resulting solution in 20X PBS. 1X PBS is prepared by dilution of 1part 20X PBS with 19 parts distilledwater. The 1X PBS solution has a pH in the range of 7.1-7.3, typically7.2, a conductivity in the range of 13,500 to 15,500 μMho-cm⁻¹ and is0.15M in NaCl.

Detailed Description

In using the method of the invention, uniform particles (the corematerial) in the size range of 0.1 to 5.0 microns are coated withgelatin or gelatin and aminodextran, and the coating is fixed by meansof a chemical fixing 8 gent. The uncoated particles have a hydrophobicor partially hydro-phobic surface. The preferred size of the particlesis in the range of 0.1 to 1.0 microns.

The magnetic particles used in the claimed invention may be preformedmagnetic particles that are dispersible in gelatin solution or they maybe magnetic particles prepared by the in situ use of gelatin in thepreparation of said magnetic particles. The in situ method for thepreparation of monodispersed colloidal particles of ferrites ofmanganese, zinc, mixed manganese-zinc, iron, barium, cobalt and nickelinvolves the use of an aqueous metal hydrodroxide gel first formed bymixing ferrous and other metal salts in an aqueous gelatin solution withpotassium or sodium hydroxide and potassium or sodium nitrate solution,all solutions being purged with nitrogen gas. The conversion of the gelto the metal oxide sol is achieved by mild thermal treatment at 90° C.(low temperature) for 4-72 hours, during which nitrate oxidation offerrous iron occurs. The magnetic particles in the hydrosol are thenwashed and resuspended in a 1% aqueous solution of gelatin of the typedescribed below or an aqueous aminodextran solution prior to furthertreatment as described herein. In preparing magnetic particles using insitu gelatin as described herein, only one type of gelatin has beenfound optimal for such use. This is type B or alkali-cured gelatin witha pI range of 4.75 to 5.0. The procedures for the preparation ofmagnetic particles using in situ gelatin are fully described incopending application Ser. No. 07/532,432, filed Jun. 4, 1990, theteachings of which is incorporated here by reference, and also describedherein. The gelatins and aminodextrans which are crosslinked accordingto the present invention are given below.

Although ferrite particles can be prepared in situ in the presence ofaminodextran as described in copending application Ser. No. 07/827,347,the interaction between the amino and the alcohol groups present on anaminodextran with the metal hydroxide and the resultant particles isweaker than the corresponding gelatin interaction. Consequently, ferriteparticles are prepared in the presence of type B gelatin and preformedparticles possessing a first layer of type B gelatin, as described inU.S. Pat. No. 5,062,991 and copending application Ser. No. 07/607,253,are suitable for use according to the present invention. Using suchparticles, e second layer of an aminodextran is coated on such particlesin place of the second coating of type A gelatin described in copendingapplication Ser. No. 07/607,253. Particles of spinal ferrites such asMnFeO₄ and ZnFeO₄ which are ferrimagnetic are preferred overferromagnetic ferrites such as magnetite (FeFe₂ O₄), CoFe₂ O₄ and NiFeO₄because the net magnetization is considerably lower in the ferrimagneticspecies due to partial cancellation of magnetic moment from twonon-equal, sub-lattice magnetic structures. Consequently, ferrimagneticspecies such as MnFe₂ O₄ and ZnFe₂ O₄, and their composite particles, inthe 0.1 to 10.0 μm diameter size range possess less residual permanentmagnetic dipole moments than magnetite particles in the same size range.This results in a lower tendency to form aggregates. Some aggregationtendency remains due to the trend to reduce the energy associated withlarge ratio of surface area to volume possessed by such small particles.

Gelatin is obtained from highly crosslinked collagen in fibrous tissue,such as skin or bone, which has been acid or base cured and thenthermally degraded at or above 39° C. The collagen molecule combines thehelical structure of the α-type proteins with the inter-chain hydrogenbonding of the β-type proteins. The three collagen peptide chains, eachin the form of a left handed helix, are twisted about each other to forma superhelix. Upon treatment, the three peptide strands of thesuperhelix are separated by the breaking of inter-chain hydrogen bondsand replacing them with hydrogen bonds to water molecules. The separatedpeptides have random coil configurations. "The Theory of thePhotographic Process", T. H. James,Ed., (New York: MacMillan Press,1977). The α-1 peptide chain has been sequenced and found to have over1000 residues. D. J. S. Hulmes et al., J. Mol. Biol., 79:137 (1973).They contain extensive segments of mainly non-polar residues; and thepolar residues which are present are not localized into acidic or basicregions. Furthermore, in contrast to globular proteins which tend toexpose their hydrophilic residues on their surfaces and bury theirhydrophobtc residues within their structure {see R. E. Dickerson et al.,"The Structure and Action of Proteins", (Menlo Park: Benjamin, 1969)},random coil gelatin has exposed hydrophobic residues readily availablefor adsorption onto the surface of hydrophobic particles such aspolystyrene latex particles or magnetite and ferrite particles. Whenaqueous gelatin is adsorbed onto the surface of s particle, itshydrophilic side chains (aspartyl, glutamyl and lysyl residues) tend tobe directed externally to the aqueous medium. The lysyl groups, whichfunction as the intramolecular crosslinkage points in collagen, will beaccessible for cross linking in the adsorbed gelatin. Glutaraldehyde isfrequently used as the crosslinking agent. U.S. Pat. No. 4,478,946 toVan Der Merwe et al. and S. B. Sato et al., a. Biochem., 100:1481-1492(1986).

A number of different, usually bifunctional, crosslinking agents such asbis 2-(succinimido-oxycarbonyloxy)-ethyl! sulfone,disuccinimidytartarate, ethylene glycol bts(succinimidylsuccinate),disuccinimidyl suberate and glutaraldehyde may be used in the claimedinvention. Glutaraldehyde, the preferred gelatin and/or aminodextrancrosslinking agent, as commercially available, contains mainly monomerabsorbing at 280 nm (nanometers). However, there is present in thecommercial product a significant amount of polymeric material whichgives rise to an absorbance at 235 nm. The polymeric species, probablytrimers or linear oligomers, are of sufficient length to form intra- andinter-molecular bridges between amino groups present on the adsorbedgelatin. By Judiciously selecting the reaction time between the adsorbedgelatin and/or aminodextran coating and glutaraldehyde, the gelatinand/or aminodextran coating can be suitably fixed on the core particlesso that it will not be removed during subsequent separation, reactionand washing steps. Large flocs created by excessive crosslinking of freegelatin and/or aminodextran can thereby be avoided and interparticlecrosslinking is negated.

Several types of gelatin are available for use in the present invention,such as type A, acid cured, isoelectric point pH 8.3-8.5 end type B,alkali cured, isoelectric point, pH 4.75-5.0. Each type is available ine variety of Bloom Numbers which indicate gel strength. Type A gelatinBloom Numbers useful in the claimed invention range from 60 to 300. TypeB Bloom Numbers useful in the claimed invention range from 60 to 225.The type A, 175 Bloom gelatin used in the preferred embodiment ofparticles having a second crosslinked gelatin layer is preferred and wasselected for its relatively large number of lysyl residues and its lowerBloom number in order to minimize intermolecular interactions betweengelatin molecules. For optimum adsorption on magnetite and ferriteparticles, it was buffered to pH 8.4, the middle of its isoelectricpoint range, at which pH it is most soluble in water and gives the leastviscous solution. The instability of gelatin adsorbed on ferriteparticles, which instability arises when glutaraldehyde is added, wasovercome by the present invention by the use of more dilute particle andgelatin concentrations 0.1% weight/volume (w/v) instead of the 2.5% w/vsolids suspension that was used in other reactions herein in conjunctionwith an inert polymeric stabilizer, polyvinylpyrrolidone (PVP), thatdoes not react with glutaraldehyde. The use of the stabilizer and the25-fold lower gelatin concentrations avoids interparticle crosslinkingduring the glutaraldehyde fixation reaction. Since polymer desorption isa very slow process relative to the time of the glutaraldehyde fixationreaction approximately 6 minutes, a stable gelatin coating around thecore particle was produced.

In order to be useful in the biological and medical arts, the fixed(crosslinked) gelatin only (2 layers) or gelatin/aminodextran coatingshould contain functional groups which can be conjugated withbiologically active substances such as antibodies to produce immobilizedbiologically active substances attached to the particle surface.Covalent coupling of biological substances to the particle surface ispreferred over simple adsorption. The coupling of an antibody, eitherpolyclonal or monoclonal, to the crosslinked gelatin or aminodextransurface is accomplished by the use of "short chain" diamines orpolyamines and a hetero-bifunctional reagent. (Hereafter, the wordpolyamine includes diamines). The polyamine is reacted with residualaldehyde or carboxylate groups, either naturally occurring or present bythe steps of this invention, present on the crosslinked gelatin surface.The use of polyamine serves not only to block aldehyde/carboxylategroups, but also serves to replenish gelatin amino groups such as lysylamino groups or aminodextran amine groups which were depleted during thecrosslinking process. This procedure is generally accomplished in twosteps. In the first step, unreacted terminal aldehyde groups of thecrosslinking agent are reacted with a polyamine followed by sodiumborohydride (NaBH₄) reduction of the resulting Schiff's base to createstable, saturated C--N linkages. In the second step, exposed carboxylicacid residues (glutamic, aspartic) of gelatin are coupled to polyaminein the presence of a water soluble carbodiimide such as1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDAC).

Short chain diamines or polyamines are preferred in order to avoidcrosslinking neighboring aldehyde or carboxylic acid groups on the sameparticle or to avoid linking such groups on different particles. Onepolyamine amine group reacts with the crosslinked gelatin oraminodextran surface and the other(s) remains unreacted and availablefor coupling, directly or indirectly, to a biological substance.Examples of `short chain` diamines or polyamines includeethylenediamine, phenylenediamine, propylenediamine,1,4-cyclohexanediamine, cyclohexenediamine, tetramethylenediamine,diethylenetriamine, 1,5-diamino-3-(2-aminoethyl)pentane (H₂ NCH₂ CH₂)₃C! and other polyamines of general formulas H₂ NCH₂ --(CH₂)_(x) --CH_(y)(CH₃)_(z) --NH₂ and C₆ H_(4+a) (NH₂)₂₁ where x=0-3, y=1 or 2 and z=1when y=1 or z=0 when y=2, and a=0 or 6. Ethylenediamine is preferred.Aminodextrans may also be used as a preferred polyamine reagents.

The coupling of the biological substance to the particle involvesactivation of the free amino groups of the coated, crosslinked particleswith a water soluble hetero-bifunctional reagent such as 2-iminothiolanehydrochloride (IT),sulfosuccinimidyl4-(N-maleimidomethyl)cyclohexane-1-carboxylate(sulfo-SMCC), m-maleimidobenzoyl-N-hydroxysuccinimide ester,N-succinimidyl-3-(2-pyridyldithio)propionate,succinimidyl-4-(p-maleimidophenyl)butyrate,N-succinimidyl-(4-iodoacetyl) aminobenzoate, the reagents listed aboveas substitutes for glutaraldehyde and the like. The 2-iminothiolanehydrochloride and the maleimidyl/succinimidyl reagents are preferred.E.Ishikawa, Immunoassay Supp., 1:1-16 (1980) and J. Immunoassay,4:209-227 (1983); M. Imagawa et al., J. Appl. Biochem., 4:41-57 (1982);and M. D. Partis, J. Protein Chem., 2:263-277 (1983). When usingsulfo-SMCC, the active sulfosuccinimidyl ester end of sulfo-SMCC willreact at pH 7.0-7.5 with amines to give peptide bonds. Thesulfo-SMCC/diamine bridging unit which results is approximately 16Angstroms in length.

When performing the polyamine and sulfo-SMCC reactions, particleaggregation was monitored by microscopic examination (1000×magnification) and by light scattering analysis using a Coulter N4MDsubmicron particle size analyzer (Coulter Corporation, Hialeah, Fla.),or similar instrument.

The maleimidyl group of sulfo-SMCC will react at pH 6.5-7.5 with freesulfhydryl groups to form a stable, covalent thioether bond. However, itis essential that the coated particles with which sulfo-SMCC is reactedcontain no free sulfhydryl groups which could react with the maleimidylend of sulfo-SMCC. Sulfhydryl groups are found on or generated fromcystine and cysteine amino acid residues of which gelatin oraminodextran have very few or none. Consequently, the crosslinkedparticles of the claimed invention do not require a protein modifier toblock free sulfhydryl groups prior to reaction with sulfo-SMCC.

Biological substances, particularly either monoclonal or polyclonalantibodies, can be covalently linked to the maleimidyl end of sulfo-SMCCfunctionalized particles by means of sulfhydryl groups present, eithernaturally or by derivatization, on said biological substances.Biological substances which have cysteinyl residues inherently containsulfhydryl groups. To introduce additional sulfhydryl groups, thebiological substances' amine groups are activated with Traut's reagent,2-iminothiolane hydrochloride (IT), at a pH in the range of 7-10. M.Erecinska, Biochem. Biophys. Res. Commun., 76:495-500 (1977); J. M.Lambert et al., Biochemistry, 17:5406-5416 (1978); and M. E. Birnbaumeret al., Biochem J., 181:201-213 (1979). When the bio-substances areantibodies, antibody lysyl and terminal amine groups are activated byIT. In the present invention, reaction conditions and the concentrationof reactants were varied to determine the optimal coupling so that thebio-substance, especially antibody, when conjugated with the substrateparticles, retains its maximum functional activity. Although maleimidesreact quite rapidly with sulfhydryl groups in solution, the same groupsimmobilized on particles were given longer reaction periods to reactwith protein. Particle and antibody concentrations during antibodyconjugation were optimized to avoid aggregation, particularly when IgMantibodies were used. The procedures optimized for IgM antibodies can beused for all monoclonal antibodies with an isoelectric point range ofabout 5.0 to about 9.0. Generally, about 30-fold less antibody wasrequired to achieve covalent coupling than is required for simpleadsorption; a consequence of importance where expensive or hard toobtain antibodies are involved.

The optimum concentration of iminothiolane-activated antibody to use inconjugation reactions with maleimidyl-activated particles was determinedby the use of activated antibody binding curves (Surface Antibody vsTotal Antibody Concentration). After a typical conjugation period, asample is taken and filtered through a 0.2 μm low-protein bindingfilter. The filtrate is analyzed spectrophoto-metrically and the surfaceantibody is determined by the difference between the total antibody inthe starting solution and the antibody in the filtrate (TotalAntibody-Filtrate Antibody). The binding data in antibody (Ab)concentration dependent runs show Langmuir isotherm-typecharacteristics; i.e., a linear low concentration region for totalantibody versus surface antibody concentration, a smooth inflectionpoint and a plateau indicating saturation at the particle surface athigh concentrations. The antibody concentrations actually used werethose at the inflection point or at concentrations slightly above theinflection point. Binding constants were obtained graphically byrecasting the equation of a hyperbole into one for a straight line. Adouble reciprocal plot of 1/n₂ ^(s) versus 1/C₂ was constructed, wheren₂ ^(s) is the number of moles of IT-Ab bound per gram of particles andC₂ is the molar concentration of free IT-Ab at equilibrium. Linear plotsare indicative of Langmuir-type binding behavior. The binding constantsK₁ =n^(s) K of IT-Ab for sulfo-SMCC-activated ferrite particles werecalculated using the equation 1/n₂ ^(s) =1/(n^(s) KC₂)+1/n^(s), where Kis the intrinsic binding constant and n^(s) is the number of moles ofbinding sites per gram of ferrite particles. Linear regression analysisof plots for various monoclonal antibodies gave the following results:

    ______________________________________                                        Ab T11:    K = 1.3 × 10.sup.6 M.sup.-1                                                          n.sup.s = 5.9 × 10.sup.-8 mol/g                 Ab KC16:   K = 6.4 × 10.sup.6 M.sup.-1                                                          n.sup.s = 5.1 × 10.sup.-7 mol/g                 Ab 1D3:    K = 2.7 × 10.sup.6 M.sup.-1                                                          n.sup.s = 2.0 × 10.sup.-7 mol/g                 Ab MO2:    K = 1.8 × 10.sup.7 M.sup.-1                                                          n.sup.s = 7.1 × 10.sup.-7                       ______________________________________                                                                mol/g                                             

The results for the ferrite particles compare favorably with similardata for commercially available carboxy-modified latex beads (23%magnetite, 0.980 μm dis., obtained from Rhone-Poulenc) covalently coatedwith aminodextran and conjugated to monoclonal antibodies end protein.These results are:

    ______________________________________                                        Ab T11:    K = 6.5 × 10.sup.5 M.sup.-1                                                          n.sup.s = 1.1 × 10.sup.-7 mol/g                 Ab KC16:   K = 3.2 × 10.sup.6 M.sup.-1                                                          n.sup.s = 6.9 × 10.sup.-8 mol/g                 Ab 1D3:    K = 3.2 × 10.sup.5 M.sup.-1                                                          n.sup.s = 1.7 × 10.sup.-7 mol/g                 Ab MO2:    K = 2.0 × 10.sup.6 M.sup.-1                                                          n.sup.s = 1.6 × 10.sup.-7 mol/g                 Ab KC48:   K = 2.5 × 10.sup.5 M.sup.-1                                                          n.sup.s = 7.6 × 10.sup.-8 mol/g                 Ab PLT-1:  K = 2.8 × 10.sup.5 M.sup.-1                                                          n.sup.s = 2.2 × 10.sup.-7 mol/g                 Streptavidin:                                                                            K = 1.3 × 10.sup.6 M.sup.-1                                                          n.sup.s = 9.5 × 10.sup.-8                       ______________________________________                                                                mol/g                                             

DESCRIPTION OF THE PREFERRED EMBODIMENTS USING IN SITU FORMED ANDPREFORMED MAGNETIC PARTICLES I. Preparation of Magnetic Particles HavingFirst and Second Gelatin Layers

Preparation of Magnetite and Other Magnetic Particles in GelatinSolution.

10 mmol (5 ml) of 2M KNO₃ solution, 12.5 mmol (2.5 ml) of 5M KOHsolution and 11.25 ml of double distilled water (DDW) were mixed andpurged with N₂ gas for 10 minutes (Solution A). 6.25 mmol (6.25 ml) of1M FeSO₄ solution and 25 ml of freshly prepared, N₂ purged, 2% type B,225 Bloom, bovine skin gelatin solution useful gelatin solution range isfrom about 0.8% to about 2.0%! were then added to Solution A in a Pyrex®bottle, mixed, swept with N₂ gas, capped tightly, and placed undisturbedin an oven at 90° C. for 4 hours. After the suspension of blackmagnetite particles had reached room temperature, they were sonicatedfor 1/2 hour, washed with 1% type B, 225 Bloom gelatin solution, andthen contacted with a large excess of 1% w/v gelatin as is the nextstep.

Metal ferrites may also be prepared using gelatin in situ in theirpreparation. In trials with other metals, namely Mn²⁺, Zn²⁺, Co²⁺, Ni²⁺,and (M²⁺), the molar ratio of M²⁺ :Fe²⁺ was kept at 1:2, but nitrate wasused instead of sulfate for Co²⁺ and Ni²⁺. The total metal-to-hydroxidemolar ratio was maintained at 1:2; but the relative KNO₃ to total metaland KNO₃ to KOH molar ratios were altered. preparing the mixed Mn/Znferrite, a 1:1 molar ratio of manganese sulfate to zinc sulfate and thesame total molar amount of non-ferrous metal ions were used. Thefollowing is an example.

10 mmol (5 ml) of 2M KNO₃ solution, 18.75 mmol (3.75 ml) of 5M KOHsolution and 6.875 ml DDW were mixed and purged with N₂ gas for 10minutes (Solution C). 6.25 mmol (6.25 ml) 1M FeSO₄ solution, 3.125 mmol(3.125 ml) of 1M Co(NO₃) solution and 25 ml of type B, 225 Bloom, bovineskin gelatin solution were mixed end purged with N₂ gas for 10 minutes.(Solution D). Solution D was added to Solution C in a Pyrex® bottle,mixed, swept with N₂ gas, capped tightly, and placed undisturbed in anoven at 90° C. for 5 hours. After the suspension of brown particles hadreached room temperature, it was sonicated for 1/2 hour, the particleswashed with 1% type B, 225 Bloom gelatin solution and then contactedwith a large excess of 1% w/v gelatin as in the next step.

Using the methods described above, cobalt and nickel ferrite particlesof about 0.1 and 0.2 μm in diameter and of spherical shape were formedin large, loosely-held brown aggregates. Zinc gave low yields of lightbrown magnetic material of less than 0.2 μm diameter even after 72 hoursof heat treatment. Dark brown manganese ferrite particles of uniform,spherical shape and 0.3 μm diameter were obtained as single particles in83-88% yields. Similar light brown manganese-zinc ferrite particles wereproduced in 49-55% yield after 72 hours of heat treatment at 90° C. Forbarium, the procedure was modified since BaSO₄ is insoluble in water.(Except for the case where barium is present, the divalent metals may beused as their chlorides or sulfates as well as their nitrates). Thus6.25 mmol (6.25 ml) of 1M FeCl₂ solution, 0.5 mmol (5.0 ml) of 0.1Ba(NO₃)₂ solution and 25 ml of 2% gelatin were mixed and purged with N₂gas for 10 minutes (Solution D). Solution C and the remainder of theferrite preparation procedure was unchanged except 10 mmol KOH solution(2 ml) was used and the heat treatment was continued for 20 hours. Blackbarium ferrite particles of uniform non-spherical shape with a 0.2 μmdiameter were produced.

Preparation of Gelatin Coated Magnetic Particles.

A quantity of magnetic particles, for example, manganese ferriteparticles, of uniform size (0.3 μm) and spherical shape and preparedusing in situ gelatin according to the procedures described above werecontacted with a large excess of 1% w/v, type B, 225 Bloom aqueousgelatin solution. Alternately, preformed (i.e., formed by methods otherthan the in situ use of gelatin), dispersible magnetic particles, forexample, manganese ferrite particles, of uniform size (0.3 μm) andspherical shape were contacted with a large excess of 1% w/v, type B,225 Bloom gelatin solution at ambient temperature for approximately 60minutes. The particles (either of the above) were then magneticallyseparated and washed five times with a 2% w/v, type A, 175 Bloom gelatinsolution in 0.2M aqueous sodium chloride, pH 8.4. After washing, theparticles were stored at ambient temperatures for up to several monthsas 2.5% w/v (weight/volume) solids suspension in a 2% w/v aqueoussolution of the type A gelatin containing 0.2M sodium chloride, 0.1% w/vsodium azide at pH 8.4. Provided the azide content of the storagesolution is maintained, the suspension can be stored for up to about 3months.

Crosslinking the Adsorbed Gelatin.

62.5 μL of 25% aqueous glutaraldehyde (0.156 mmol) solution were addedto 120 ml of 1% aqueous polyvinylpyrrolidone (MW:=40,000) in 0.2Maqueous sodium chloride, pH 7.2. To this, 5 ml of the 2.5% solidsuspension prepared above were added to the glutaraldehyde solution andthe resulting suspension was mixed at ambient temperature for a time inthe range of 3-15 minutes, preferably about 6 minutes.

Blocking of Unreacted Aldehyde Groups.

0,105 ml of 99% ethylenediamine (1.56 mmol) was added to a 125 mlsuspension of the fixed, gelatin coated magnetic particles (0.1%w/vsolids) in 1% PVP solution, 0.2M in sodium chloride, pH 7.2. Theresulting suspension was mixed for a time in the range of about 1 to 4hours, preferably about 2 hours, in a 250 ml tissue culture flask. Atthe end of the mixing time, 1.25 ml of a 10 mg/ml solution of sodiumborohydride (NaBH₄) in 0.1 mM KOH were added to the magnetic particlesend the resulting suspension was mixed for an additional 15 minutes. Theparticles were then magnetically separated and washed a plurality,preferably three, times with 0.2M aqueous sodium chloride.

Reaction with Fixed Gelatin's Carboxylate Residues.

2.11 ml of 99% ethylenediamine were added to an 118 ml suspension of thealdehyde-blocked beads, 0.1% w/v solids, in 0.2M aqueous NaCl. Theresulting suspension was physically and sonically mixed forapproximately 15 minutes. After this mixing, 4.5 ml of 10 mg/ml EDAC in0.2M NaCl was added and the suspension was first physically andsonically mixed for approximately 15 minutes, and finally physicallymixed for a time in the range of about 8-16 hours. The contents of theflask were then magnetically separated, washed a plurality of times with1X PBS, sonically mixed in 1X PBS for approximately 30 minutes, andfinally concentrated to 5 ml of 2.5% w/v solids in 1X PBS. For largescale (100×) preparations, the previous aldehyde blocking step and theEDAC coupling step have been combined to avoid multiple separations andwashings. The combination of steps did not result in any loss ofactivity in the final antibody-conjugated beads.

Crosslinking the Adsorbed Gelatin Without PolyvinylpyrrolidoneStabilizer Present.

5 ml of 2.5% w/v solids manganese ferrite particles suspended in 2% w/vtype A, 175 Bloom gelatin in 0.1M phosphate buffer, pH 8.4, which wereprepared as described above were magnetically separated. The clearsupernatant liquid was discarded and the residue of magnetic particleswas resuspended in 5 ml of 3 mg/ml glutaraldehyde solution prepared bymixing 56 μL of 25% aqueous glutaraldehyde solution with 5 ml of 1 mMaqueous potassium hydroxide, pH 10.00. The resulting suspension ofmagnetic particles was mixed, preferably roller mixed, for about 30minutes. After the glutaraldehyde addition and mixing was completed,about 34 μL of ethylenediamine (10:1 diamine-to-glutaraldehyde molarratio) were added to the reaction mixture which was then stirred for anadditional 2-3 hours. Subsequently, about 0.313 ml of a 40 mg/mlsolution of sodium borohydride in 1 mM KOH was added to the reaction andthe resulting mixture stirred for about 10-30 minutes. The crosslinkedparticles were then washed three times using magnetic separation andresuspended in 5 ml of 1 mM aqueous potassium hydroxide.

Activation of Diamine Treated Particles with Sulfo-SMCC.

In general, 27 μL of freshly prepared 10 mg/ml sulfo-SMCC in 1X PBS wereused per milliliter of 2.5% w/v magnetic particle suspension. In atypical preparation, 135 μL of the sulfo-SMCC solution were added to 5ml of 2.5% w/v particles. The mixture was then roller mixed in a 15 mlplastic centrifuge tube for approximately one hour, sonically mixed forapproximately 5 minutes, magnetically separated, and washed a pluralityof times with 1X PBS.

The functionalized, crosslinked, gelatin coated particles resulting fromthe above series of steps have pendent maleimidyl groups end aresuitable for a variety of medical end/or biological uses. If thesubstance which is desired to be conjugated to the particles has asufficiency of active sulfhydryl groups, activation of that substance isnot necessary, and the following step may be skipped.

Antibody Activation with 2-iminothiolane Hydrochloride.

A 51.24 mg/ml concentrate of T11 monoclonal antibody in 1X PBScontaining 0.1% NaN₃ was prepared. For 10 mg of T11 antibody and 15mg/ml antibody concentration during coupling, the total reaction volumeshould be 0.667 ml. Using a 15:1::IT:T11 activation ratio, 0.9375 μmol(0.129 mg) IT (65 μL of 2mg/ml IT) in 1X PBS is required. Therefore,0.407 ml of 1X PBS solution was added to 0.195 ml of T11 concentrate, towhich resulting solution an additional 65 μL of 2mg/ml IT solution wasadded. The net resulting solution was roller mixed in s tube reactor for1 hour. The content of the reaction tube was then applied to the top ofa 20 ml G-0 Sephadex column, equilibrated and washed with 100 ml 1X PBS.The derivatized antibody was eluted using 1X PBS and e plurality of 2.5ml fractions were collected with the aid of a UV monitor. Fractions inthe middle of the band absorbing at 280 mm were pooled and the A₂₈₀value was used to determine T11/=T antibody concentration. Typically,the T11/IT concentration was about 3.0 mg/ml. The T11/IT solution may beconcentrated by solvent removal.

Conjugation of T11/IT with Sulfo-SMCC Derivatized Particles.

In a laboratory scale conjugation, total volume 5 ml, the concentrationof particles was 2.5% w/v solids and the T11/IT concentration was 0.9mg/ml. In one sample, when the purified T11/IT solution concentrationwas 1.850 mg/ml, then 2.392 ml of T11/IT antibody solution in 1X PBS wasadded to 5 ml of 2.5% w/v solids sulfo-SMCC activated particles whichhad been preconcentrated by the removal of 2.432 ml of supernatant. TheT11/IT solution was added to the particles in 0.5 ml increments withsonic and rapid physical mixing between additions. The resultantsolution was then roller mixed in a 15 ml tube for approximately twohours. A 1 ml test sample was then taken, filtered through a low-proteinbinding 0.2 μm filter, and the filtrate analyzed spectrophotometricallyfor T11 antibody by measuring the absorbance at 280 nm; A₂₈₀ =c(supernatant)=0.3986 mg/ml. Measurement by difference, c (surface)=c(total)-c (supernatant)!. Thus c (surface)=0.9 mg/ml-0.3986 mg/ml=0.501mg/ml. This translates to a T11 surface loading of 20 mg T11 per gramparticles or, for a specific surface area of 4.89 m² /g for manganeseferrite particles, a 4.1 mg T11/m² particle surface area. Similarprocedures with 2-and 3-fold dilutions of particle concentration, butthe same total antibody concentration during conjugation, gave highersurface antibody loading. However, a limitation was reached when a4-fold dilution of the particles concentration did not yield highersurface coverage of antibody.

Blocking Unreacted Maleimidyl and Sulfhydryl Groups.

Unreacted maleimidyl groups on the sulfo-SMCC activated particles wereblocked with L-cysteine after antibody conjugation. Typically, 0.480 mlof 5 mg/ml L-cysteine in 1X PBS was added to remaining 4 ml of theconjugation mixture of the previous step and the resulting solution wasroller mixed for 15 minutes. Unreacted sulfhydryl groups were blocked bythe addition of 0.534 ml of 20 mg/ml iodoacetamide in 1X PBS followed bythe addition of 0.100 ml of 1M, pH 9.8 sodium borate buffer solution.The resulting solution was roller mixed for 30 minutes, the blockedconjugation mixture was magnetically separated and the particles washedthree times with 1X PBS containing 1% bovine serum albumin (fraction V,heat shock) and 0.1% NaN₃ (BSA buffer solution). After washing, 4 ml ofthe foregoing BSA solution were added to the particles, the particlesroller mixed for approximately 1hour, stored at 4° C. for a time in therange of about 8-16 hours, magnetically separated and washed threeadditional times with BSA buffer.

Antibody containing particles prepared according to the method describedherein have been found useful in various cell separation assays. Thebiological substances used in assays utilizing the Invention may beselected from the groups consisting of normal or non-normal T-cells,B-cells, leukocytes, viruses, erythrocytes, cells of the breast, uterus,colon, kidney, liver, lung, testes, stomach, thyroid and parathyroid,and the like; provided that the biological substance contains anantigenic determinant capable of binding to an antibody.

In an embodiment of the invention equivalent to the magnetic particleembodiment described above, the maleimidyl groups and the sulfhydrylgroups are transposed. That is, the crosslinked gelatin coated particlesare derivatized to have pendent groups ending in reactive sulfhydrylgroups in place of the maleimidyl groups described above and theantibodies are derivatized to have reactive maleimidyl groups in placeof the sulfhydryl groups described above. The methods used to preparethis equivalent embodiment are the same as described above. In bothcases, the antibody is connected to the gelatin surface by a molecularbridge prepared as described.

The following examples are given to illustrate the utility of theclaimed invention and are not to be taken as limiting said invention.

EXAMPLE 1 Protocol for Magnetic Bead Depletion of T-cell and B- cellPopulations.

Mononuclear cells (MNC) were obtained from whole blood samples bydensity isolation on Ficoll-hypaque gradients and washed with 1X PBS.1×10⁶ MNC in 1 ml 1X PBS were added to a series of tubes containing 5,10, 25, 50 and 100 μL of the monoclonal antibody (mAb) conjugatedmagnetic particle suspension (2.5% w/v) being tested. Two tubes were setup for each depletion and for the undepleted control. The resultingsuspensions were then mutated for 3 minutes in a multi-tube vortexer ora single tube mutator. At the end of incubation, the cell suspension wasplaced for a total of 2 minutes in the magnetic field provided by asingle tube magnetic rack. At the end of the magnetic separation,unbound cells were extracted by withdrawing all the clear liquid fromthe center of the tube with a Pasteur pipet.

For T- or B-cells (T11, T3, T4, T8, B1, B4), the cell suspensioncollected after depletion was compared directly to the original cellsuspension prior to particle depletion. The samples, original anddepleted, were centrifuged for 5 minutes et 1200 rpm end the supernatantdecanted to leave approximately 100 μL of 1x PBS remaining in each tube.One tube of each pair of depletion tubes was then stained with 10 μLCYTO-STAT® MsIgG1-RD1/MsIgG1-FITC control reagent (MS) and the othertube was stained with 10 μL CYTO-STAT® T11-RD/B4-FITC reagent (for T11,T3, B1 or B4 depletions) or with 10 μL of T4-RD1/T8-FITC reagent (for T4or T8 depletions) at room temperature for 10 minutes. At the end ofincubation, 500 μL of 1X PBS were added to each sample and the sampleswere analyzed by flow cytometry. The samples were analyzed on the EPICS®Profile using the MBead 2-Color program. (EPICS® and CYTO-STAT® areregistered trademarks of Coulter Corporation). As the original samplestained with Ms control reagent was being run, it was checked todetermine whether the lymphocyte population was fully incorporated inBitmap 1, and adjustments were made if necessary. The left side ofdiscriminator 2 was set for each fluorescence histogram on the channelwhich would give <1% positive staining. This was done for each samplestained with Ms control reagent and then the corresponding tube stainedwith specific antibody was analyzed. The data were collected andrecorded as the absolute number of positive staining cells in the redand green histograms (T and B or T4 and T8) not percent positive. Testresults are summarized below.

EXAMPLE 2 Protocol for Magnetic Bead Depletion of Red Blood Cells (RBC).

100 μL of Na4EDTA-anticoagulated whole blood were placed in a series ofreaction tubes. To each tube, 25 to 150 μL of KC-16 conjugated magneticparticles suspension (2.5% w/v) were added and the total volume wasadjusted to 250 μL using 1X PBS. The suspension were nutated for 3-5minutes in a multitube vortexer or a single tube nutator at low mixingspeed. When nutation was completed, 1 ml of 1X PBS was added to eachsample tube which was then placed on a magnetic rack for 2-5 minutes.All the supernatant was removed from each tube using a Pasteur piper andsaved in labelled tubes. Samples were analyzed on a Coulter S-plus® IVor similar rbc counter as total rbc number/m1 whole blood. The positivecontrol was 100 μL whole blood plus 1,150 ml 1X PBS to give 100% rbccount and the negative control was 100 μL whole blood plus 1.150 ml ofBatch lyse or similar lysing agent to give 0% rbc count. Percentage ofrbc depleted=100% (rbc count in sample tube)/(100% rbc count )!.

EXAMPLE 3 Protocol for Magnetic Bead Depletion of Leukocytes

100 ml of Na₄ EDTA-anticoagulated whole blood were collected, dividedamong 8 number of centrifuge tubes and centrifuged at 500 g for 10minutes. The majority of plasma was removed and the buff colored layerof cells from each tube was removed, pooled together and centrifuged at500 g for an additional 10 minutes. The buff colored cells and theplasma constitute the leuko-rich whole blood which should have an rbccount no greater than 8.0 ×10₉ /ml and a white blood cell (wbc) of2-4×10₇ /ml.

100 μL of leuko-rich whole blood were pipetted into a number of reactiontubes. An amount of 10 to 160 μL of magnetic bead suspension (2.5% w/v)was then pipetted into each tube followed by the addition of 200 μL of1X PBS. (N.B. Lower titer points with 10 to 40 μL of beads should be runfirst. Additional beads were added only if endpoint depletion was notobtained at 40 μL). Each tube was nutated for 3-5 minutes at low speed.2 ml of 1X PBS were then added, the contents of a tube mixed and thebeads then magnetically separated for 2 minutes. All supernatant liquidwas removed and placed in a duplicate tube which was then centrifuged at400 g for 5 minutes. The resulting supernatant was then carefullyremoved by pipette and analyzed.

The leuko-rich or the leuko-depleted whole blood samples were analyzedby the addition of 10 μL of single or dual color antibody preparationdesigned to discriminate for the depletion of specific cells from amixture of cells. For example, when T11-conjugated magnetic beads wereused in depletion, T11-B4 dual color was used to discriminate betweenactual T11 cell depletion and the non-specific depletion of T11 cells(i.e. B cells). The mixture was vortexed and incubated for 10 minutes atroom temperature in the dark. Controls were isotype control and antibodycontrol with undepleted cells. The tubes were then placed on a CoulterEPICS® Q-prep, or similar instrument, and run on the 35 seconds lysemode. After the rbcs were lysed and the samples fixed (Q-prep), allsamples were analysed on a Coulter EPICS® Profile flow cytometer orsimilar instrument. This procedure is required to obtain data as actualnumber of cells per volume of sample. Programs available on Profile wereused to analyze lymphocyte and monocyte-myeloid populations.

Summary of Test Results using the Protocols of Examples 1-3.

1. In a T11/B4 lymphoid cell assay, the undepleted control gave 97,209T11+, 18,240 B4+, 19,717 monocyte and 25,381 granulocyte counts. Afterdepletion with 10 μL of 2.5% w/v solids magnetic beads conjugated withT11 antibody, the counts were 15,826, 20,181, 19,954 and 30,972respectively. Depletion with 20 μL T11 antibody conjugated beads gave2,256, 20,989, 20,874 and 31,965 counts; 30 μL gave 1,150, 21,428,20,697 and 35,362 counts; and 40 μL gave 644, 21,232, 19,817, and 33,935counts, all respectively.

2. In a T4/T8 lymphoid cell assay, the undepleted control, whichcontained 4.1×10⁵ T8 and 7.9×10⁵ T4 cells, gave 54,415 T4 and 27,906 T8counts. After depletion with 10, 20 and 30 μL of 2.5% w/v/solidsmagnetic beads conjugated with T8 antibody the counts were 57,030 and12, 59,538 and 6, and 60,905 and 5, respectively.

3. In an erythrocyte/thrombocyte assay, the undepleted control contained4.5×10⁶ wbc, 4.4×10⁸ rbc and 4.7×10⁷ platelets. Depletion experimentswere conducted using 20, 40, 60 and 80 μL of 2.5% w/v solids magneticbeads conjugated with KC-16 antibody. The wbc, rbc and plateletsremaining after depletion were 20 μL: 4.4×10⁶ wbc, 1.6×10⁸ rbc and4.3×10⁷ platelets; 40 μL: 4.6×10⁶ wbc, 1×10⁷ rbc and 4.5×10⁷ platelets;60 μL: 4.5×10⁶ wbc, 1×10⁷ rbc and 4.3×10⁷ platelets; and 80 μL: 4.5×10⁶wbc, 1×10⁷ rbc end 4.3×10⁷ platelets. The results indicate that 40 μL of2.5% solids beads which contained 1.85×10¹⁰ particles removed 4.3×10⁸rbc, thus giving a particle-to-rbc ratio of 43.

4. In a myeloid cell assay, the undepleted control gave 73,821lymphocyte, 13,426 monocyte and 55,661 granulocyte counts. Depletionstudies were conducted using 10, 20, 30 and 40 μL of 2.5% w/v solidsmagnetic beads conjugated with KC-48 antibody. The results were 10 μL:70,330, 9,309 and 340 counts; 20 μL: 68,414, 2,006 and 1,332 counts 30μL: 62,966, 1,597, and 922 counts; and 40 μL: 59,340, 1,546 and 899counts, all respectively.

A similar depletion study was conducted using 10, 20, 30 and 40 μl of of2.5% w/v solids magnetic beads conjugated with 1D3 antibody. The resultswere 10 μL: 76,405, 13,839 and 1,597 counts; 20 μL: 73,198, 8,653 and1,216 counts; 30 μL: 65,667, 2,590 and 2,130; and 40 μL: 66,276, 1,906and 1,686 counts, all respectively.

A further depletion study was conducted using 10, 20, 30 and 40 μL of2.5% w/v solids magnetic beads conjugated with MO2 antibody. The resultswere 10 μL: 72,563, 3,107 end 56,520 counts; 20 μL: 72,905, 3,616 and34,533 counts; 30 μL: 69,644 1,618 and 32,313 counts; and 40 μL: 69,477,1,210 end 30,899 counts, all respectively.

5. In an erythrocyte/thrombocyte assay, the undepleted control contained7×10⁶ wbc, 4.9×10¹⁰ rbc and 3.0×10⁷ platelets. Depletion studies wereconducted using 20, 40, 60 and 80 μL of 2.5% w/v solids magnetic beadsconjugated with PLT-1 antibody. The results, after depletion, were 20μL: 10×10⁶ wbc, 5.4×10¹⁰ rbc and 1×10⁶ platelets; 40 μL: 10×10⁶ wbc,5.8×10¹⁰ rbc and 1×10⁶ platelets; 60 μL: 7×10⁶ wbc, 5.1×10¹⁰ rbc and1×10⁶ platelets; end 80 μL: 10×10⁶ wbc, 5.6×10¹⁰ rbc and 0 platelets.

II. Preparation of Magnetic Particles Having A First Gelatin Layer and aSecond Aminodextran Layer

Preparation of Aminodextrans.

Method A. Small scale preparation of aminodextran.

Aminodextran was prepared by partial cleavage and oxidation of theglycopyranose rings in dextran to aldehyde functional group, coupling ofthe aldehyde groups with 1, 3-diaminopropane to form Schiff baselinkages and reduction of the Schiff's base linkages to form stablecarbon-nitrogen bonds. In a typical procedure, 20 g of dextran weredissolved in 150 ml of 50 mM potassium acetate buffer, pH 6.5. Asolution of 2.14 g of sodium peroiodate in 25 ml of distilled water wasadded dropwise to the dextran over about 10 minutes with vigorousmagnetic mixing. The resulting solution was stirred at room temperature,15-27° C., for about 1.5 hours and then dialyzed against distilledwater. 20 ml of 1,3-diaminopropane were mixed with 20 ml of distilledwater, cooled in an ice bath, vigorously stirred and pH adjusted fromabout 11.5 to about 8.7 over about 15 minutes by the addition of glacialacetic acid. Typically, 15-20 ml of glacial acetic acid was used. Thedialyzed dextran solution was added dropwise over about 15-20 minutes tothe chilled diamine solution. After the addition was completed, theresulting solution was stirred at room temperature for about 2.25 hours.A reducing solution of 0.8 g sodium borohydride in 10 ml of 0.1 mMsodium hydroxide was added to the dextran reaction mixture at roomtemperature over about 15 minutes. The reaction mixture was stirredduring the borohydride addition to expel most of the effervescence. Thecrude aminodextran solution was exhaustively dialyzed against distilledwater until the conductivity of the effluent was 3-4 μmho/cm. Thedialyzed solution was then filtered through an 0.2 μm filter andfreeze-dried over 24 hours in a model TDS-00030-A, Dura-Drymicroprocessor controlled freeze-dryer (FTS Systems, Inc.) to produce4.25 of flaky, pale yellow crystals in 21% yield.

Method B. Large scale preparation of aminodextran.

The procedure of Method A was modified for the large scale prepartion ofaminodextran and for increasing the number of amino groups introducedinto dextran. Hollow fiber membrane filtration replaces dialysis and asmaller diamine-periodate molar ratio was used to avoid further cleavageof the sugar polymer into lower molecular weight fragments. A hollowfiber cartridge (polysulfone, 3 ft³ membrane surface area, 1 mm diameterfibers and 5,000 MW cut-off) was mounted vertically with an input powerpump (two pump heads, maximum flow rate of about 4.56 liters/minute withNo. 18 Norprene® food grade tubing) delivering 15-20 psi whichcorresponds to 5-10 psi in the retentate line. The filtrate wascollected at 50-100 ml/min. Washing was done using 20-30 liters ofdistilled water over about 6-8 hours. The specific conductance wasreduced to about 3-4 μmho-cm and the pH was 6.0-6.5. The feed volume wasmaintained at 2 liters during desalting and then concentrated to 800 mlin the first washing of oxidized dextran and to 400 ml in the secondwashing of aminodextran.

In a standard scaled-up preparation, 80 g of dextran were transferred to1 quart liter! glass blender bowl containing 600 ml distilled water. Thesolid was blended for about 2-5 minutes at medium speed to dissolve allthe dextran. 8.56 g of sodium periodate were dissolved in 100 ml ofdistilled water and the resulting solution was added dropwise to thedextran solution over about 10 minutes using vigorous magnetic stirring.After the addition was completed, the resulting mixture was stirred atroom temperature for an additional 3 hours. The resulting viscousreaction mixture was then diluted to 2 liters with distilled water anddesalted using a hollow fiber cartridge. The initial specificconductance was 1.5 mmho-cm⁻¹ or higher and the initial pH was 4.0.About 18-22 liters of distilled water were used to obtain solutionhaving a final pH of 6.0-6.5. The final volume of washed, oxidizeddextran solution was 800 ml.

To the washed, oxidized dextran solution, 80 ml of colorless, liquid1,3-diaminopropane were slowly added over about 10 minutes at roomtemperature. The resulting mixture was then stirred at room temperaturefor an additional 3 hours. After the stirring was finished, 3.2 g ofsodium borohydride dissolved In 40 ml of 1 mM aqueous sodium hydroxidewere added to the room temperature aminodextran reaction mixture overabout 5 minutes with magnetic stirring. After the completion of thesodium borohydride addition, the resulting mixture was stirred for anadditional 1 hour end then desalted using a hollow fiber cartridge. TheInitial specific conductance was 5.0 mmho-cm⁻¹ or higher and the initialpH was about 12.0. About 20-25 liters of distilled water were needed toreduce the specific conductance to about 3-4 μmho-cm⁻¹ and the pH to6.0-6.5. The final volume of aminodextran solution was 400 ml. Thissolution was passed through a 0.2 μm sterile cellulose acetate filterunit and then freeze-dried over 48 hours to obtain 48 grams of flaky,pale yellow crystals, a 52% yield.

Elemental analysis (C,H,N) were obtained for two samples of aminodextranprepared from dextran T-2M by the methods described above. The analysesare:

Sample 1.20 g dextran scale, desalting by dialysis. Obsd.: C, 43.04; H,6.60, N, 1.09; 0 (by difference), 49.27.

Sample 2.80 g dextran scale, desalting by membrane filtration. Obsd.: C,42.53; H, 6.52 N,1.01; 0 (by difference), 49.94 Calculated for C₄₆ H₇₉NO₃₇. 3H₂ O: C, 42.76; H, 6.63; N, 1.08; O, 49.53

The analyses for aminodextran in the two preparations were very similar,thus indicating that the same product was obtained whether desalting wasdone by dialysis or by membrane filtration. The empirical formulaobtained for Sample 1, C₄₆ H₈₄ NO₄₀, is very similar to the formula C₄₆H₇₉ NO₃₇.3H₂ O based on 29 units of glucose (C₆ H₁₀ O₅), 1 unit of fullydiamine-substituted sugar ring (C₁₂ H₂₈ N₄ O₃) and twelve units ofwater. Therefore, the degree of diamine substitution in dextran was 1/30in Sample 1 in contrast to a theoretical value of 1/12 based on 100%periodate cleavage and diamine substitution. The empirical formulaobtained for Sample 2, C₄₉ H₉₀ NO₄₃, is very similar to the formula C₄₉H₈₄ NO₄₀.3H₂ O based on 31 units of glucose, 1 unit of fully diaminesubstituted sugar ring and twelve units of water. The degree ofsubstitution in dextran by diamine was 1/32 for Sample 2.

In the preparation of aminodextran coated particles, similar resultswere obtained using aminodextrans having average molecular weights of10,000, 40,000 and 2,000,000 (T-10, T-40 and T-2M) with 1X (1X=3.3%substitution of sugar residues), 2X (6.6%) 3X (9.9%) and 5X (16.5%)molar amounts of amino groups. All the aminodextrans were initiallyprepared according to methods A and B using 2 and 3 times the amount ofsodium periodate used in the 1X oxidation of dextran. The amount of1,3-diaminopropane used for Schiff base formation was kept constant.

Modifications have been made to the Methods A and B of preparingaminodextrans which were originally disclosed in Ser. NO. 07/827,347.These modifications involve the oxidation and cleavage of the dextranglucose rings with periodate anion, diamine addition and sodiumborohydride reduction of the Schiff's base. The modifications have ledto increased yield on the aminodextrans, particularly the 5X-aminodextran. Generally, the first modification was to use only a tenpercent (10%) excess of diamine over the stoichiometric 2:1diamine:periodate molar ratio previously disclosed. Second, the diamineaddition reaction was conducted at a temperature in the range of about5-10° C. Third, the diamine addition reaction was spectroscopicallymonitored in the near ultraviolet (UV) region for Schiff base formation.Schiff's base formation was deemed completed when successive spectralanalyses indicated e plateau was reached. The reaction was then stopped.These modifications reduced aminolysis of the polymeric sugar groupsinto lower weight fragments and thus gave higher yield of product afterpurification and concentration by hollow fiber membrane filtration. Thehollow fiber filtration was done using polysulfone cartridge of 3 ft²membrane surface area, 1 mm diameter fiber having a 5,000 molecularweight cut off. The cartridge was mounted vertically in an input powerpump having two pump heads delivering 15-20 psi with s maximum flow rateof 4.56 liter/minute when using No. 18 Norprene® food grade tubing. Withthis configuration, the pressure in the retenate line was about 5-10psi. The filtrate was collected at 50-100 ml/min. Washing was done using20-30 liters of distilled water over about 6-8 hours. The followingmethod for preparing 5X -aminodextran is given to illustrate themodified procedure which is applicable to the preparation of allaminodextrans.

Method C. Preparation of 5X -Aminodextran.

T-2M dextran (50 g, 0.308 mol, obtained from Sigma or Pharmacia) wasadded to a 1-quart or 1-liter glass blender bowl containing 300 ml ofdistilled water. The mixture was blended at maximum speed until all thedextran dissolved, typically about 3-5 minutes. A solution of 26.75 g(0.125 mol) of NaIO₄ in 300 ml distilled water was added to dextransolution over about a 10 minute period using vigorous magnetic stirring.After the periodate addition was completed, the reaction mixture wasstirred at room temperature for about an additional three hours. Afterthe three hours, the 600 ml reaction volume had an specific conductivityof 9.7 mmho-cm⁻¹ and an initial pH of 2.5. The reaction mixture wasdiluted to two liters distilled water and desalted using the hollowfiber cartridge. Washing was done using 15-18 liter of water to obtain600 ml of washed, oxidized dextran solution having a specificconductance of 10 mmho-cm⁻¹ end pH of 6.7.

The solution of oxidized dextran was cooled to about 8° C. using an icebath and 23.2 ml (0.275 mol) of diaminopropane was added over about 10minutes to the oxidized dextran solution. The resulting reaction mixturewas stirred and maintained at the ice bath temperature. The formation ofthe yellow Schiff's base was monitored ever 10-15 minutes by measuringthe 335 mm near UV absorbance of an extracted smaple. In a typicalexperiment, the measurements at 335 nm using a lmm path length cellwere:

                  TABLE 1                                                         ______________________________________                                        minutes     absorbance values                                                 ______________________________________                                        0           0.100                                                             5           2.063                                                             15          2.975                                                             30          3.692                                                             45          3.901                                                             60          4.103                                                             75          3.784                                                             ______________________________________                                    

After the absorbance had reached a plateau, 19.3 g (0.500 mol) of sodiumborohydride in 19.3 ml of 1 mM aqueous potassium hydroxide were added tothe reaction mixture over about 10 minutes at ambient temperature withmagnetic stirring. After the sodium borohydride addition was completed,the reaction mixture was stirred at ambient temperature for about anadditional two hours. After the stirring was completed, spectroscopicmeasurement at 335 nm using a 1 cm path length cell gave an absorbancevalue of 0.067 units which indicates that the Schiffis base compound hadessentially disappeared. The reaction mixture, about 1000 ml volume, wasthen desalted using the hollow fiber cartridge. The initial specificconductance was 43 mmho-cm⁻¹ and the initial pH was 11.0. About 18-20liters of distilled water were used as wash liquid to produce about 300ml of 5X -aminodextran solution having a specific conductance of about4-6 μmho-cm⁻¹ and a pH of 6.5-7.0. The 5X -aminodextran solution wasfiltered through a 0.2 μm cellulose nitrate filter and freeze-dried over48 hours in a model TDS-00030-A, Dura-Dry® microprocessor-controlledfreeze-dryer (FTS Systems, Inc.) to produce 24 g (48% yield) of flaky,pale yellow crystals. Elemental analysis: C=45.83%, H=7.00%, N=4.49%, O(by difference)=42.68%. Calculated analysis for C₁₂ H₂₂ O₈.25 N:C=46.15%, H=7.10%, N=4.48%, 0=42.26%.

The empirical formula based on actual analysis is C₁₂ H₂₂ O₈.3 N, whichis very similar to the formula C₁₂ H₂₂ O₈.25 N based on 6 units ofglucose per one unit of fully dismine-substituted sugar ring (C₁₂ H₂₈ N₄O₃). Therefore, the degree of diamine substitution in dextran was 1/7 incontrast to a theoretical value of 1/2.5 based on 100% periodatecleavage and diamine substitution. Repeat experiments at 200 g and 300 gdextran scales produced a product having the same degree ofsubstitution.

Preparation of Magnetite and Other Magnetic Particles An GelatinSolution.

Metal ferrites (MFe₂ O₄ where M=Fe²⁺, Mm²⁺, Zn²⁺, Co²⁺, Ni²⁺ and Ba²⁺)including magnetite can be prepared in situ in type B gelatin asdescribed in I above. To further illustrate the invention, manganeseferrite was prepared as follows.

62.5 mmol (62.5 ml) of 1M Fe₂ SO₄ solution, 100 mmol (50 ml) of 2M KNO₃solution, 31.25 mmol (31.25 ml) of 1M MnSO₄ solution and 72.92 ml ofdistilled water were mixed together in a one liter Pyrex® bottle andpurged with N₂ gas for 10 minutes (Solution A). Then, 166.6 mmol (33.33ml) of 5M KOH was added to Solution A, swept with N₂ gas and sonicatedat room temperature (18-27° C.) for a time in the range of about 5-10minutes to yield a smooth, dark green slurry of Fe(OH)₂ gel. Aftersonication, 250 ml of 2% type B, alkali cured, 225 Bloom bovine skingelatin solution were added to the Fe(OH)₂ gel, mixed, swept with N₂gas, capped tightly, sonicated for about 5-10 minutes and placedundisturbed in a 90° C. oven for about 24 hours. After oven heating, thePyrex® bottle and contents were removed from the oven and allowed tocool to room temperature. After cooling was completed, the suspension ofbrown particles in the bottle was mixed and decanted into two 250 mltissue culture flasks. The fraction in each flask was washed about fivetimes with 1% w/v type B gelatin or with 1% 1X aminodextran solutionusing magnetic separation between washings. After washing, the magneticparticles were recombined, suspended in sufficient 1% 1X -aminodextransolution to make 500 ml total volume and sonicated for about 0.5 hour.The particles were stored at refrigeration temperatures of about 5-100C. for 8 time of 8 hours to at least six months. Storage was done as a2.5% w/v solids suspension in 1X -aminodextran solution containing 0.1%w/v sodium azide. Alternatively, the particles may be used immediatelyafter preparation in type B gelatin and washing with 1X -aminodextran.

Preparation of 5X -Aminodextran Coated Magnetic Particles.

Method 1. A quantity of stored manganese ferrite particles preparedaccording to the procedure described above and of uniform 0.3 μm sizeand spherical shape were washed a plurality of times with distilledwater, dried at 105° C. and the w/v solids in suspension was determined.A sample which analyzed as having 0.56% w/v solids was used herein.

100 ml of 0.56% w/v solids, gelatin prepared, aminodextran-storedmanganese ferrite particles were magnetically separated and resuspendedin an equal volume of 2% w/v 5X -aminodextran solution. The 100 mlsuspension of brown particles was sonicated for about 10 minutes andthen mixed overnight (about 8-16 hours) in a 250 ml tissue culture flaskon an orbital shaker. After the mixing was completed, the pH wasadjusted to 10.0 with about 20 μL of 5M aqueous potassium hydroxide. Theadsorbed 5X -aminodextran was crosslinked by the addition of 1.173 ml of25% aqueous glutaraldehyde (3.11 mmol) to the pH adjusted particlesuspension and mixing the resulting suspension for a time in the rangeof 1-4 hours, preferably one hour, using an orbital shaker. Thecrosslinked particles were magnetically separated, the supernatantmeasured and discarded, and, to block and stabilize unreacted aldehydegroups, the particles resuspended in a volume of 1% 5X -aminodextransolution equal to that of the discarded supernatant. Alternatively, apolyamine solution such as ethylenediamine or 1,3-diaminopropanesolution may be used to block the unreacted aldehyde groups. The molaramount of polyamine is about ten times the amount of glutaraldehydeused. The resuspended particles were shaken and sonicated to dispersethem, and the resulting suspension was mixed overnight using an orbitalshaker. A 23.5 ml sample of 10 mg/ml sodium borohydride (6.22 mmolNaBH₄) in 1 mM aqueous potassium hydroxide was then added to thesuspension and the resulting mixture orbitally mixed for an additional0.5 hour. The particles were then magnetically separated and washed aplurality of times (minimum three) with 1X PBS and concentrated to 22.4ml volume to yield a 2.5% w/v solids suspension.

Method 2. A 60.2 ml sample of 0.83% w/v solids manganese ferriteparticles suspended in 1% w/v 1X -aminodextran solution containing 0.1%sodium azide was magnetically separated and washed a plurality of timeswith 20 ml portions of 0.2M aqueous sodium chloride. After washing, theparticles were resuspended in sufficient 3.75 mg/ml 5X -aminodextran in0.2M sodium chloride solution (about 20 ml) to give a 2.5% w/v solidssuspension. Next, for each milliliter of the 2.5% w/v solids suspension,9 μL of 10 mg/ml EDAC.HCl in 0.2M sodium chloride solution was added tothe suspension (180 μL total). The resulting mixture was orbitallyshaken in a 50 ml tissue culture flask for a time in the range of about12-16 hours. The particles were magnetically separated, washed aplurality of times with distilled water and resuspended in 1X PBS toyield 20 ml of 2.5% w/v solids suspension of magnetic particle having acoating formed by a condensation reaction between gelatin and theaminodextran. This condensation reaction occurs between aminodextranamine groups and carboxylate groups present on the gelatin. EDAC removesthe water which is formed during the condensation reactions as a urea.The use of a crosslinking agent such as glutaraldehyde is not necessary.The resulting particles are defined as being equivalent to particleshaving a first gelatin layer, a second aminodextran layer andcrosslinked by a chemical crosslinking agent.

Determination of Crosslinked Gelatin and 5X -Aminodextran LayerThickness.

A. Gelatin Thickness.

A glutaraldehyde crosslinking solution was prepared by mixing 56 μL of25% aqueous glutaraldehyde with 5 ml of 1 mM aqueous potassiumhydroxide. The particles from 5 ml of a 2.5% w/v solids suspension ofmanganese ferrite particles prepared in type B gelatin as described andsuspended in 2% w/v, type A, 175 Bloom gelatin in 0.1M phosphate buffersolution, pH 8.4, were magnetically separated and the supernatant liquiddiscarded. The separated particles were resuspended in 5 ml of theglutaraldehyde solution and mixed, preferably using a roller mixer, forabout 30 minutes. The particles were again magnetically separated andwashed three times with 5 ml of 1 mM aqueous potassium hydroxide beforeresusupension in 5 ml of 1 mM aqueous potassium hydroxide.

Elemental analysis was performed using 5 ml of crosslinked, gelatincoated manganese ferrite particles that were washed fifteen times withdistilled water and dried to constant weight at 1100° C. Analyticalresults were: Mn=19.05%, Fe=49.49%, C=0.54%, H<0.5%, N<0.5% and O (bydifference)=30.92%. The percentage of carbon weight in gelatin can beobtained from its amino acid content (The Theory of the PhotographicProcess, 4th T. H. James, ed (Macmillan, New York 19767), Chapter 2,page 52. The type A gelatin used herein gives C=50.46%, H=6.765%,N=18.27%, O=24.29% and S=0.21%. Using this information, the gelatinlayer thickness can be calculated.

Using 1 g of gelatin coated ferrite particles, there will be 1g×0.0054/0.5406=0.01070 g gelatin and 0.9893 g ferrite. The particlevolume of a 0.29 μm diameter manganese ferrite sphere is 1.277×10⁻¹⁴ cm³and the number of particles in 0.9893 g manganese ferrite is 1.827×10¹³.Consequently, the mass of gelatin per ferrite particle is 5.856×10⁻¹⁶ g.

Assuming a density of gelatin coating of 0.02 g/cm from the 2% w/vgelatin before crosslinking with glutaraldehyde, the volume of gelatinper particle is 2.92×10⁻¹⁴ cm³. As a result, the total volume, gelatinplus ferrite, per particle is 4,205×10⁻¹⁴ cm³ and the radius of agelatin-ferrite sphere As 2,157×10⁻⁵ cm (0.2157 μm). The thickness ofthe gelatin coating on the manganese ferrite sphere is thus 0.2157μm-0.145 μm (the mean radius of the ferrite sphere)=0.0707 μm (71 mm).This is in good agreement with the values of 750 Angstroms on glassgiven A. T. Kudish et al., Proteins at Interfaces, ACS Symposium Series343, J. L. Brash et al., eds. (American Chem. Soc., Washington, D.C.1987), pages 261-277; 75 nm on mica given by N. Kawanishi, et al., J.Phys. Chem. 94:4611-4617 (1990); and 600-700 Angstroms on glass given byH. Metzer et al., J. Colloid Interface Sci. 126:292-303 (1988).

The gelatin layer thickness was similarly calculated for manganeseferrite particles where the gelatin layer was crosslinked usingpolyvinylpyrrolidone (PVP) stabilizer. Elemental analysis gave:Mn=18.84%, Fe=47.82%, C=1.67%, H<0.5, N<0.5 and O (by difference)=31.67.The calculated thickness of the gelatin coating was calculated as 148mm. The thicker coating using PVP is believed to result from proceduraldifferences. The thicker coating was obtained when the ferrite-gelatinsolution is diluted, in the original procedure, from 2% to 0.08% and 1%PVP is added. The thinner coating was obtained when the gelatin-ferriteparticles were separated from the 2% gelatin solution and resuspended inthe crosslinking medium. As a result, there was no excess polymer,either gelatin or PVP, present.

B. Aminodextran Thickness.

Manganese ferrite particles were coated with 5X-aminodextran (Method 1)which was subsequently crosslinked, blocked end stabilized. Theresulting particles were washed a plurality of times with distilledwater, magnetically separated and dried at 110° C. Elemental analysisresults were: Mn=14.04%, Fe=44.36%, C=2.97% and O (bydifference)=38.63%. For 5X -aminodextran analyzed as containing 45.83% Cand a 2% w/v 5X -aminodextran coating, the estimated aminodextran layerhas a thickness of 218 mm.

Activation of Diamine Treated Particles with Sulfo-SMCC.

The same procedures described for activating gelatin coated particleswith sulfo-SMCC are used to activate the 5X -aminodextran coatedparticles except that five times the activating amount of sulfo-SMCCwere used. When the magnetic particles are coated with 1X -, 2X- or3X-aminodextran, one, two or three times the amount of sulfo-SMCC isused, respectively.

Antibody Activation with 2-Iminothiolane Hydrochloride.

Antibodies were activated with 2-iminothiolane hydrochloride accordingto the procedures previously described herein. If the antibody or othersubstance which is to be conjugated to the activated particles has asufficiency of reactive sulfhydryl group, activation by 2-iminothiolanehydrochloride may not be necessary. Those skilled in the art will alsorecognize that the activation of particles to contain maleimidyl groupsand the activation of antibodies or other substances can be switched.That is, the particles can be activated by the introduction of reactivesulfhydryl groups and the antibody or other substance can be activatedto contain reactive maleimidyl groups.

The following example is given to illustrate the utility of theinvention and are not to be taken as limiting the invention. Unlessotherwise indicated, Method 1 was used to coat particles with 5X-aminodextran in this example, two methods for enumerating neutrophilsin a leukocyte-rich (leuko-rich) whole blood sample are described. Themonoclonal antibody 1D3 conjugated to 5X -aminodextran coated beads andflow cytometry is used in both methods.

In the first method, a series of 1D3 containing magnetic bead titerswere prepared, mixed with the leuko-rich blood sample and thenmagnetically separated from the sample. The neutrophil-depletedsupernatants were then analyzed by flow cytometry to determine the pointat which neutrophil depletion was complete. Samples were analyzed in theorder of lowest titer to highest titer and depletion was recognized by aconstant cytometer count.

In the second method, whole blood samples were mixed with the sametiters of 1D3-conjugated magnetic beads, but no magnetic separation wasperformed. The mixtures were analyzed by flow cytometry for neutrophilsshifted out of the normal granulocyte region in the forward versus side(orthogonal) scatter histogram because of s change in cell size, shapeor refractive index due to attachment of magnetic beads to theneutrophil cell surfaces. When the shifted neutrophil population orcount reached a plateau value, its count in the light scatter histogramor its fluorescent event count in the bitmapped region was compared tothe respective counts for all white blood cells (lymphocytes, monocytesand granulocytes) to obtain the relative number of neutrophils.

The use of well-defined ferrite particles of uniform size, uniformspherical shape and uniform refractive index properties is essential inobtaining a recognizable, definitive shift in light scatter frombiological cells conjugated to the surface of particles such as theparticles described herein. In addition to electric dipole scatteringarising from chemical substances, magnetic dipole scattering from theferrite particles can make a substantial contribution to the intensityof the light scattering arising from the cell-particle conjugates J. A.Stratton, Electromagnetic Theory (McGraw Hill, New York 1941), page 437,and C. F. Bohren and D. R. Huffman, Adsorption and Scattering of Lightby Small Particles (Wiley, New York 1983), page 141!. J. J. Murry,Optics 4:1011 (1965), has measured low angle light scattering bymagnetic particles such as magnetite.

The diameter of the manganese ferrite particles which are used hereinwas 0.29±0.08 μm. This diameter lies outside the mean diameter range of0.65-3.0 μm for the polystyrene latex and magnetic latex particles whichwere used in International Patent Application Publication WO 90/13013entitled METHOD AND APPARATUS FOR SCREENING CELLS OR FORMED BODIES WITHPOPULATIONS EXPRESSING SELECTED CHARACTERISTICS UTILIZING AT LEAST ONESENSING PARAMETER and PCT Application No. PCT/US/08590 (to CoulterCorporation, the assignee herein) entitled METHOD AND APPARATUS FORSCREENING NICROSCOPIC CELLS UTILIZING LIGHT SCATTER TECHNIQUES. Thepolystyrene latex particles used in these studies were of uniform sizeand shape. However, while the magnetic latex particles, which wereformed by embedding ferrofluid particles of varying size and shapeinside the latex during emulsion polymerization (U.S. Pat. No. 4,358,388to Daniel et al.), were spherical in shape, they varied widely in size.For example, while the mean particle diameter was 0.7 μm, individualparticles varied from 0.2 μm to 1.0 μm. (An additional problem arisesfrom the fact that the structure of the ferrofluid particles embedded inpolystyrene latex is not definable.) The small diameter particles,though low when measured by weight percent, were very numerous.Consequently, due to the higher mobility of the smaller particlesrelative to the mobility of the larger particles, the smallerparticle-antibody conjugates will preferentially occupy antigenic siteson cell surfaces. However, the contribution of these small particlestoward shifting the light scatter of cells is minimal and the use ofthese magnetic particles to obtain a true enumeration or count oftargeted shifted cell populations is unreliable. This may, in part, bedue to a resultant low net magnetic dipole when a summation is carriedover many magnetic particles of different spacial orientation within thelatex particles.

In addition to their uniform size and uniform spherical shape, the indexof refraction of dense, microcrystalline ferrite particles that areformed and coated as described here is sufficiently different from theindex of refraction of polystyrene latex and biological cells. Whenselected cell sets or subsets are coupled to these magnetic particles,large shifts occur in the forward versus side scatter histograms of theconjugated cells relative to the histogram of unconjugated cells.Consequently, cells conjugated to the particles can be distinguishedfrom those not conjugated to particles.

EXAMPLE 4. Enumeration of Neutrophils

Preparation of Samples.

A 50 ml sample of Na₄ EDTA-anticoagulated whole blood was divided amonga plurality of centrifuge tubes and centrifuged at 500g for about 10minutes. The majority of the plasma was removed and the buff coloredlayer of cells in each tube was removed, pooled and centrifuged again at500 g for about 10 minutes. Collectively, the buff colored cells and theplasma constitute the leuko-rich whole blood which should have a redblood cell (rbc) count no greater than 8.0×10₈ /ml and a white bloodcell (wbc) count between 2-4×10₇ /ml.

100 μL of the leuko-rich buff colored cells were pipetted into aplurality of reaction tubes. A quantity of 5 to 250 μL of 2.5% w/vsuspended solids, 1D3-conjugated, 5X -aminodextran coated manganeseferrite particles prepared as described herein was pipetted intoseparate tubes. Typically the titers of ferrite particle suspension were0, 5, 10, 20, 30, 40, 50, 75, 100, 150, 200 and 250 μL. The volume ofeach tube was then made up to 350 μL by the addition of 1X PBS. Eachtube was nutated for 6 minutes at low speed and the contents of eachtube were magnetically gathered and separated for 60 seconds. Thesupernatant liquid which constitutes a leukocyte-depleted(leuko-depleted) sample was carefully removed by pipette end placed induplicate tube for analysis.

Leuko-rich (undepleted) and leuko-depleted buff colored cell sampleswere flow cytometrically analyzed to discriminate between actualneutrophil depeletion and non-specific neutrophil depletion; that is,depletion of lymphocytes, monocytes, eosinophils and basophils inaddition to neutrophils. Each sample undergoing analysis was treated bythe addition of 10 μl of the single color antibody reagent CYTO-STAT®KC56-FITC (Coulter Corporation, Hieleah, Fla.), vortexed and incubatedfor 10 minutes at room temperature. The control samples were isotypecontrol and antibody control conjugated to undepleted buff coloredcells. The tubes were then placed in a Counter EPICS® Q-prep or similarinstrument and processed on the 35 second lyse mode. After the rbcs were1ysed and the samples fixed (Q-prep), the total volume of each samplewas 1,100 μL. All samples were then analyzed on a Coulter EPICS® ProfileII flow cytometer or similar instrument. The programs available on theProfile II instrument were used to analyze lymphocyte andmonocyte-myeloid populations as fluorescent events. To estimate thenumber of cells per volume of sample, the wbc count of the undepletedsample was determined on a Coulter S-Plus® counter (4.19×10⁶ wbc), 100μL sample aspiration, and was compared with the total fluoresecentevents measured (1.81×10⁶) for lymphocytes, monocytes end granulocytesin the undepleted sample and 100 μL sample aspiration. As s result ofthese measurements, the fluorescent events determined for the depletedsamples were scaled by a factor of 2.31 (4.19×10⁶)+(1.81×10⁶)! and adilution factor of 11 to obtain actual cell counts.

Procedure 1. Flow Cytometry Analysis for Neutrophils After Magnetic BeadDepletion of Samples.

A myeloid cell assay of an undepleted control sample resulted in 29,131lymphocyte, 19,282 monocyte and 116,479 granulocyte counts asfluorescent events. The granulocytye percentage in the total wbcpopulation is therefore 70.6%. Results obtained using the CoulterS-Plus® counter gave a granulocyte total of 71.7%. Depletion analysiswas done using 5, 10, 20, 30, 40, 50, 75, 100, 150, 200 and 250 μL of0.25% w/v solids magnetic beads conjugated with 1D3 monoclonal antibody.The results (Table 2) were:

                  TABLE 2                                                         ______________________________________                                               Counts                                                                 μL Beads                                                                            lymphocytes   monocytes                                                                              granulocytes                                  ______________________________________                                        5        32,428        11,638   106,421                                       10       32,243        9,971    86,017                                        20       32,200        5,477    42,296                                        30       33,884        4,178    31,505                                        40       30,788        2,166    23,008                                        50       30,707        1,597    16,117                                        75       29,914        1,373    13,471                                        100      28,327        961      10,645                                        150      24,427        701      8,127                                         200      21,127        512      7,147                                         250      16,933        470      6,397                                         ______________________________________                                    

Both neutrophils and monocytes were depleted throughout the titer with1D3-conjugated magnetic beads. Monocyte removal can be avoided orsubstantially reduced by shortening the nutation time for the buff celland magnetic bead mixture from 6 minutes to 30 seconds and reducing themagnetic separation time from 60 seconds to 15 seconds. Based on thedifference between undepleted and depleted sample fluorescent events forgranulocytes and the total wbc fluorescent events in the undepletedsample, the percentage of neutrophils in the total wbcs was 66.8%.

The neutrophil depletion data were converted to bead and cell valuesusing a value of 4.18×10¹⁰ manganese ferrite particles/ml of 0.25% w/vsolids suspension and the factor 25.41 (the fluorescent event scalefactor 2.31 times the dilution factor 11, both obtained above) toconvert fluorescent events to granulocyte cell counts. These resultingvalues (Table 3) are:

                  TABLE 3                                                         ______________________________________                                               No. Magnetic No. Neutrophils                                                                           Bead-to-Cell                                  μL Beads                                                                          Beads × 10.sup.8                                                                     Depleted × 10.sup.6                                                                 Ratio × 10.sup.2                        ______________________________________                                        5      2.1          0.26        8.1                                           10     4.2          0.77        5.5                                           20     8.4          1.9         4.4                                           30     12.6         2.2         5.7                                           40     16.8         2.4         7.0                                           50     21           2.6         8.1                                           75     31.5         2.6         12.1                                          100    42           2.7         15.6                                          150    63           2.8         22.5                                          200    84           2.8         30                                            250    105          2.8         37.5                                          ______________________________________                                    

FIG. 1, which graphically depicts the number of neutrophils depletedversus the magnetic bead-to-cell ratio, shows that a plateau inneutrophil depletion was reached between the 100 μL and 150 μL 0.25% w/vmagnetic bead titers when 2.8×10⁶ neutrophils were completely depletedusing a bead-to-cell ratio of about 2000:1. This represents 66.8% of thetotal wbc count of 4.19×10⁶ determined by the Coulter S-Plus® counter.

Procedure 2. Flow Cytometry Analysis for Neutrophils Shifted by MagneticBeads.

Leuko-rich buff cell-magnetic bead mixtures were prepared as describedin the Preparation of Samples for analysis using a Coulter Profile IIcytometer, but no magnetic separation was done. Programs were introducedinto the profile to analyze lymphocyte and monocyte-myeloid populationsin the forward versus side scatter histogram; in particular, thoseneutrophils shifted out of the normal granulocyte region due to a changein size, granularity or refractive index brought about by binding theneutrophils to the magnetic ferrite particles. The series of histogramsin FIG. 2 show a distinct and progressive shift toward less forwardscatter (FS) or smaller size and more side scatter (LSS) or greatergranularity in the normal granulocyte region as higher titera ofmagnetic beads were added to the cell samples.

The refractive index and magnetic properties of ferrite particles can,however, upset the usual trend to larger size (greater FS) and greatergranularity (larger LSS) expected as particles are attached to targetedcells. Monocytes were shifted in the same direction as the granulocyteswith the end result that at the highest magnetic bead titers theyoccupied the region normally occupied by granulocytes. Lymphocytesremained unshifted until titers greater than 100 μL of 0.25% solidsmagnetic beads were used. At these higher titers, lymphocytes wereapparently removed because there is a shift out of the lymphocyte boxedregion of the forward versus side scatter histogram, This apparent shiftoccurs when there is present an excess of magnetic beads not bound toeither monocytes or neutrophils. The excess beads appear to disturb thegranularity of lymphocytes not by binding them, but by back-scatter fromthe excess magnetic ferrite particles. The following Table 4 gives theoriginal granulocyte count, the shifted granulocyte count and thepercent of granulocytes shifted using different titer of 0.25% w/vsolids, 1D3 conjugated, 5X -aminodextran coated manganese ferriteparticles.

                  TABLE 4                                                         ______________________________________                                               Granulocyte                                                            μL Beads                                                                            Original Count                                                                             Shifted Count                                                                            % Shifted                                    ______________________________________                                        0        120,977      10,413     0                                            5        121,892      16,537     11.9                                         10       115,195      31,918     21.7                                         20       87,148       55,773     39.0                                         30       84,669       60,156     41.6                                         40       73,559       74,978     50.5                                         50       54,482       72,032     56.9                                         75       16,987       80,129     82.5                                         100      17,449       88,145     83.5                                         150      14,766       87,669     85.6                                         200      9,720        84,772     89.7                                         250      7,396        76,362     91.2                                         ______________________________________                                    

For the 100 μL titer, the total analysis results were 26,604 lymphocytescounts, 1,045 monocytes counts 17,449 unshafted granulocytes counts and88,145 shafted granulocyte counts. As a result, the percent neutrophilsin the total wbc count was calculated as 66.2%. The neutrophilpercentage obtained by the population shaft method agrees well with the66.8% calculated from the magnetic depletion data obtained usingProcedure 1.

To further illustrate the utility of the invention, additionaldepletieon studies where performed on (a) red blood cells and platelets,and (b) white blood cells.

EXAMPLE 5 Magnetic Bead Depletion of Red Blood Cells and Platelets.

100 μL of Na₄ EDTA-anticogulated whole blood were placed in a pluralityof reaction tubes. Titers of 20-160 μL of 2.5% w/v solids, KC-16conjugated, 5X -aminodextran coated manganese ferrite particles wereadded to the reaction tubes and the total volume in each tube wasadjusted to 260 μL with 1X PBS. The resulting mixtures were nutated for6 minutes in either a multi-tube vortexer or a single tube nutator atlow mixing speed. When nutation was completed, the beads in each tubewere magnetically separated for 60 seconds. The supernatant liquid wasremoved using a Pasteur pipette and was saved An iabelled tubes. Thesamples were analyzed using a Coulter S-Plus® or similar rbc counter asthe total rbc count per 100 μL of sample (whole blood plus beads plus 1XPBS). The positive control was 100 μL whole blood plus 160 μL 1X PBS togave 100% rbc count. The percentage rbcs depleted=100%- rbc count insample tube)+(100% rbc count)!.

The following Table 5 summarizes the results of anerythrocyte/thrombocyte assay. The 0 μL data is the undepleted controlsample. The depletion was done using 1.25% w/v solids, KC-16 conjugated,5X -aminodextran coated beads. KC-16 monoclonal antibody binds only toerythrocytes and does not bind leukocytes or platelets.

                  TABLE 5                                                         ______________________________________                                               Counts                                                                 μL Beads                                                                            wbc × 10.sup.5                                                                       rbc × 10.sup.8                                                                   platelets × 10.sup.6                     ______________________________________                                        0        2.1          1.71     7.2                                            20       2.1          1.34     7.1                                            40       2.1          0.89     7.1                                            60       2.3          0.54     7.8                                            80       2.4          0.36     8.1                                            100      2.4          0.25     8.0                                            120      2.5          0.15     8.5                                            140      2.3          0.04     8.5                                            160      2.3          0.03     8.3                                            ______________________________________                                    

The number of magnetic beads/ml for a 2.5% w/v solids suspensions is4.18×10¹¹. Using FIG. 3 a plot of the number of rbcs depleted versusbead-to-cell ratio, 160 μL of 1.25% w/v solid beads contained 3.36×10¹⁰magnetic particles and removed 4.37×10⁸ rbcs, thus giving aparticle-to-(bound rbc) ratio of 77.

The assay was repeated for magnetic beads coated with 5X-aminodextran byMethod 2. The assay was performed using 5, 10, 20, 40, 60, 80, 100, 150,200 and 250 μL titers of 2.5% w/v solid, KC-16 conjugated, 5X-aminodextran coated manganese ferrite particles. The positive controlwas 100 μL of whole blood in 250 μL 1X PBS which was subsequentlydiluted to 1.000 ml with 1X PBS. The wbc, rbc and platelet counts forthe undepleted control are listed in the following depletion Table 6 as0 μL beads.

                  TABLE 6                                                         ______________________________________                                               Counts                                                                 μL Beads                                                                            wbc × 10.sup.4                                                                       rbc × 10.sup.7                                                                   platelets × 10.sup.6                     ______________________________________                                        0        8            4.7      1.8                                            5        8            4.5      1.9                                            10       8            4.4      1.8                                            20       8            37       1.8                                            40       8            2.3      1.7                                            60       7            1.5      1.8                                            80       7            0.9      2.1                                            100      6            0.6      1.8                                            150      6            0.4      1.8                                            200      6            0.2      1.7                                            250      6            0.1      1.8                                            ______________________________________                                    

A plot of number of rbcs depleted versus particle-to-cell ratio willhave a plateau similar to that shown in FIG. 2. 150 μL of 2.5 w/v solidsmagnetic particles, at the plateau, contained 6.3×10¹⁰ particles andremoved 4.3×10⁸ rbcs for a particle-to-rbc ratio of 147.

A depletion assay for platelets was performed and 10, 20, 30, 40, 50,100, 150 and 200 μL of 0.83% w/v solids, PLT-1 monoclonal antibody(Coulter Corporation, Hialeah, Fla.) conjugated, 5X -aminodextran coatedmanganese ferrite particles. Total volume was adjusted to 350 μL by theaddition of 1X PBS as needed. The positive control was 100 μL of wholeblood diluted with 250 μL of 1X PBS. The wbc, rbc and platelets for theundepleted control are listed in the following depletion Table 7 as 0 μLbeads.

                  TABLE 7                                                         ______________________________________                                               Counts                                                                 μL Beads                                                                            wbc × 10.sup.5                                                                       rbc × 10.sup.8                                                                   platelets × 10.sup.6                     ______________________________________                                        0        1.7          1.51     5.2                                            10       1.6          1.48     3.1                                            20       1.6          1.51     1.2                                            30       1.5          1.50     0.4                                            40       1.4          1.55     0.3                                            50       1.3          1.45     0.2                                            100      1.1          1.57     0.1                                            150      1.1          1.46     0.1                                            200      1.0          1.49     0.1                                            ______________________________________                                    

The results as plotted in FIG. 4 indicate that 100 μL of 0.83% w/vsolids beads containing 1,39×10¹ particles removed 1.79×10₇ platelets,yieldlng a particle-to-platelet ratio of 779.

EXAMPLE 6. Magnetic Bead Depletion of White Blood Cells.

The preparation of leuko-rich samples, their depletion with KC-56monoclonal antibody (Coulter Corporation) conjugated, 5X -aminodextrancoated manganese ferrite particles and the analysis of the supernatantliquids after magnetic depletion according to Procedure 1 are similar tothose described for neutrophil depletion using 1D3 antibody-conjugatedmagnetic beads. The depletion assay was conducted using 5, 10, 20, 40,60, 80, 100, 150, 200 and 250 μL of 0.833% w/v solids, KC-56 conjugated,5X -aminodextran coated manganese ferrite particles. The 1lymphocyte,monocyte and granulocyte counts for the undepleted control sample, givenas fluorescent events, are listed in the following depletion Table 8 as0 μL beads.

                  TABLE 8                                                         ______________________________________                                               Counts                                                                 μL Beads                                                                            lymphocytes   monocytes                                                                              granulocytes                                  ______________________________________                                        0        25,461        11,730   40,740                                        5        15,660        6,447    53,165                                        10       13,746        5,801    60,708                                        20       3,474         3,589    50,268                                        40       1,266         1,362    30,370                                        60       251           123      6,147                                         80       100           83       4,333                                         100      89            5        201                                           150      16            0        8                                             200      6             1        4                                             250      0             0        1                                             ______________________________________                                    

To determine the number of cells per volume of sample, the wbc count(2.86×10⁶) of the undepleted sample was determined for a 100 μLaspiration on a Coulter S-Plus® counter and this count compared with thetotal fluorescent events (8.57×10⁵) for lymphocytes, monocytes andgranulocytes in an undepleted sample and 100 μL aspiration on theCoulter EPICS® Profile II flow cytometer. As a result, the fluorescentevents were scaled by a factor of 3.34 that is, (2.86×10⁶)+(8.57×10⁵)!and the dilution factor of 11 to obtain cell numbers. Bead numbers werecalculated using 1.39×10¹¹ particles/ml for a 0.833% w/v solidssuspension. The following depletion Table 9 tabulates the results asnumber of wbc depleted ×10⁵, number of magnetic beads using in thedepletion ×10⁸ and the bead-to-cell ratio ×10³.

                  TABLE 9                                                         ______________________________________                                                 WBC × 10.sup.5                                                                      No. Beads × 10.sup.8                                                                Bead-to-Cell                                 μL Beads                                                                            Depleted    Used in Depletion                                                                         Ratio × 10.sup.3                       ______________________________________                                        5        5.54        6.97        1.26                                         10       6.48        13.9        2.15                                         20       11.1        27.9        2.52                                         40       16.5        55.8        3.38                                         60       26.2        83.6        3.19                                         80       27.0        111         4.13                                         100      28.5        139         4.89                                         150      28.6        209         7.30                                         200      28.3        279         9.74                                         250      28.6        348         12.2                                         ______________________________________                                    

FIG. 5 is a plot of the number of wbcs depleted versus the magneticbead-to-cell ratio. FIG. 5 shows a steep rise for the bead-to-cellvalues of 2.15×10³ and 4.13×10³, and a plateau starting at 100 μL of0,833% w/v bead titer where 2.85×10⁶ wbcs were depleted at abead-to-cell ratio of about 4900:1. The removal of the largergranulocytes cells begins after almost all of the 1lymphocytes andmonocytes were depleted. This is consistent with the requirement ofneeding a greater number of magnetic beads attached to a granulocyte tomagnetically remove these cells. That is, for magnetic separation ofgranulocytes it is necessary to have more magnetic particles attached toeach of the larger granulocyte cells in order to have sufficientmagnetic moment to physically pull the cells from solution toward themagnetic field. There is no separate inflection point shown in FIG. 5for granulocyte depletion. However, some granulocytes were shafted inthe forward versus side scatter histograms for higher titers and Justprior to complete depletion. This indicates the presence of somegranulocytes that were not removed by magnetic separation, but remainedin suspension with significant numbers of magnetic beads attached to thecells.

We claim:
 1. A method of preparing aminodextran compounds in aqueous solution comprising:(a) partially oxidizing a dextran compound to dextran aldehyde using a chemical oxidizing agent such as sodium periodate at ambient room temperature; (b) desalting the dextran-aldehyde solution of step (a); (c) reacting the dextran-aldehyde of step (b) with a diamine at a temperature of about 5-10° C. to form a plurality of Schiff's base linkages between the dextran-aldehyde and amino groups of the diamine; and (d) reducing the Schiff's base linkages of step (c) to obtain an amino-dextran through the use of a chemical reducing agent.
 2. The method of claim 1 wherein said dextran has a molecular weight in the range of 10,000 to two million daltons.
 3. The method of claim 1 wherein said diamine is 1,3-diaminopropane.
 4. The method of claim 1 wherein the diamine to periodate used has a molar ratio of greater than 2.2:1.
 5. The method of claim 1 wherein the diamine to chemical oxidizing agent used has a molar ratio of about 2.2:1.
 6. The aminodextran compounds produced by the method of claim
 1. 7. The aminodextran compounds produced by the method of claim 1 wherein said compounds have a degree of sugar substitution in the range of about 3.3% to about 16.5%.
 8. A method of preparing aminodextran compounds having about 3.3% to about 16.5% of sugar residues doubly-substituted by diamine groups, said method comprising:(a) adding an aqueous periodate solution to an aqueous dextran solution over about a ten minute time period with vigorous stirring during the addition and stemming the resulting reaction mixture at room temperature for about 1.5 to about 3 hours to obtain a mixture wherein the dextran is partially oxidized to dextran aldehyde; (b) desalting and washing the mixture of step (a) to obtain a solution of the polymer dextran-aldehyde; (c) cooling the solution of polymer of step (b) to a temperature of about 5-10° C., adding a diamine to the cooled solution over about a ten minute time period and reacting the resulting mixture for a sufficient time to form Schiff's base, the duration of the reaction being determined by spectroscopic monitoring of the Schiff's base formation and the molar amount of diamine added to the reaction mixture being greater than 2.2 times the molar amount of periodate used in step (a); (d) reducing the Schiff's base by reaction with borohydride anion in aqueous hydroxide at room temperature, the duration of said reaction being determined by spectroscopically monitoring the disappearance of the Schiff's base to yield a reduced aminodextran; and (e) purifying and drying the polymeric product of step (d) to obtain said aminodextran.
 9. The method of claim 8 wherein said dextran has a molecular weight in the range of about 10,000 to two million Daltons.
 10. The method of claim 8 wherein the diamine and periodate are used in a molar ratio of about 2.2:1.
 11. The method of claim 8 wherein said diamine is 1,3-diaminopropane.
 12. The method of claim 8 wherein said temperature is in the range of 5-10° C.
 13. The method of claim 8 wherein the product of step (d) is purified by dialysis or hollow fiber membrane filtration to optimize yield. 